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In an introductory course in biology a text-book is helpful 
because the notes taken by lower class students are usually 
unsatisfactory, if not useless, and require much time. This 
volume contains an outline of a course given by the author for 
more than ten years. It is designed to supplement the practical 
work in the laboratory and field, and to relieve the student of 
the greater part of the burden of taking lecture notes. Purely 
discriptive matter is reduced to a minimum and examples are 
largely omitted because the teacher is supposed to have suffi- 
cient command of the subject to supply these, and local and 
familiar examples are always better for illustration than those 
less well known. The figures in the book are regarded as an 
important part in the presentation of the subject and should 
be carefully studied. Many points omitted or only briefly 
alluded to in the text are explained by them. 

Part I is designed to acquaint the student with the funda- 
mentals of plant organization and life processes. Part II 
does the same for animals but in a different and more thorough 
method. Part III discusses the most important general biolog- 
ical phenomena. The difference in treatment of plants and 
animals rests on well-known pedagogical principles which 
need not be discussed here. There is little in the book for 
which originality can be claimed, but justification for publishing 
rests on the fact that there is at present no text which even 
approximately covers the field in subject-matter and method 
of treatment. 

For the classification of plants and animals given in the ap- 
pendices to Parts I and II, many authorities have been con- 


suited but none followed consistently. In the interest of the 
beginner simplicity is desirable and to this end a number of 
unimportant, obscure or aberrant groups have been omitted 
and only three grades, or orders of groups, recognized, viz., 
Branch or Phylum, Class, and Order. The classification of 
animals given in the "Lehrbuch" of Claus-Grobben has been 
followed more closely than that of any other author. 

Acknowledgment is due the publishers, Messrs. P. Blakis- 
ton^s Son & Co., for the loan of many figures from the works 
of Galloway, Stevens, Folsom and others. The Macmillan Co. 
and Henry Holt and Co. have also kindly furnished several 
figures. More specific acknowledgment is made in connection 
with each borrowed figure. 





Biology and the biological sciences i 

The living and the not-living '. . 2 

The living substance protoplasm 4 


Laboratory and field exercises 7 

Color. The light relation. The leaves. Phyllotaxy. The stem. 

The roots 15 

Seeds. Germination. The seedling. The mature plant. Structure 
and function of the roots. Structure and function of the stem. 
Structure and function of the leaves. Photosynthesis. Res- 
piration. Translocation of food substances. Other food sub- 
stances. Differentiation of tissues 22 

Modified roots. Modified stems and branches. Modified leaves . . 49 
Homology of the flower. Inflorescence. Structure of the flower. 
Function of the flower. Pollination and fertilization. The 

seed. The fruit. Seed distribution 53 

Classes of plants : Angiosperms, Gymnosperms, Cryptograms ... 71 
Ecology: Water, temperature, latitude and altitude, light, soil, re- 
lation of plants to each other, carnivorous plants, physiographic 
relations 75 


Branch I. Thallophyta 101 

Class i. Myxomycetes 101 

2. Schizophyta 102 

3. Diatomeae 105 

4. Conjugate 105 




5. Chlorophyceae 106 

6. Characeae 106 

7. Phaeophyceae 107 

8. Rhodophyceae 107 

9. Phycomycetes 108 

10. Basidiomycetes 108 

11. Ascomycetes 109 

Lichenes no 

Branch II. Bryophyta in 

Class i. Hepaticae in 

2. Musci 112 

Branch III. Pteridophyta 113 

Class i. Filicinae 114 

2. Equisetinae 115 

3. Lycopodinae 115 

Branch IV. Spermatophyta 116 

Class i. Gymnospermae 117 

2. Angiospermae 117 


Laboratory exercises 119 

Introduction: Color and form. Locomotion. Axis of locomotion. 
Cephalization. Dorsal and ventral. Right and left. Bilateral 
symmetry. Radial symmetry. Universal symmetry. Asym- 
metry. Exceptional cases. Size and differentiation. Integu- 
ment. Nerve-muscle Mechanism. Digestion. Circulation. 
Respiration. Excretion. Reproduction. Organization of the 
body. "Higher" and "lower" animals. Segmentation meta- 
meres, antimeres 127 

Integument: General integument of amoeba, hydra, worms, arthro- 
pods, vertebrates. Specialized integumentary structures cu- 
ticular, epidermal and dermal structures, glands 141 

Sense organs: General sense organs in amoeba, hydra, worms, arthro- 
pods, vertebrates. Organs of special sense: The chemical 
senses taste and smell. The organs of sight. The arthropod 
eye. The vertebrate eye. Types of vision. Mechanism for 
focusing and control of light intensity. Hearing and equilibra- 
tion. Statocysts. The ear of arthropods, of vertebrates. 
The senses of lower animals 151 



Organs of response: In amoeba, hydra, annelids, arthropods, verte- 
brates. Skeleton and connective tissue. The endoskeleton of 

vertebrates. Muscular action 172 

The nervous system: Ccelenterates. Annelids. The mechanism of re- 
sponse. Arthropods. Vertebrates. The brain and spinal cord. 

A spinal nerve. The cranial nerves 182 

Energy relations of the animal: The food of animals a source of 
energy. Digestion in amoeba. Fermentation and digestion. 
Digestion in ccelenterates, in worms, in arthropods. The diges- 
tive tract of vertebrates. Digestive ferments. Absorption. . . 191 
Circulation: The gastro-vascular cavity of ccelenterates. Circulation 

in worms, in arthropods, in vertebrates. The lymphatic system. 204 
Respiration: in minute animals, in aquatic animals, in insects, in 

vertebrates. The blood as respiratory vehicle 208 

Metabolism: growth, secretion and excretion, muscular activity . . 213 
Excretion: in minute animals, in worms, in crayfish, in vertebrates . 215 
Reproduction: amoeba, conjugation, hydra, annelids, crayfish, verte- 
brates 217 



Phylum i. Protozoa: Classes of protozoa 224 

Phylum 2. Ccelenterata: Porifera. Structure of a sponge. Hy- 

drozoa. Scyphozoa. Anthozoa. Ctenophora 230 

Phylum 3. Scolecida: Platyhelminthes. Aschelminthes. Nemer- 

tini 224 

Phylum 4. Annelida: Classes of annelids . . 247 

Phylum 5. Molluscoidea : Bryozoa. Brachiopoda 248 

Phylum 6. Echinodermata: Pelmatozoa. Asteroidea. Structure 

of star fish. Ophiuroidea. Echinoidea. Holothuroidea ... 250 
Phylum 7. Arthropoda: Branchiata. The orders of Crustacea. 
Palaeostraca. Arachnoidea. Protracheata. Myriapoda. Aptery- 
gogenea. Insecta. Structure of an insect. The orders of 

insects 259 

Phylum 8. Mollusca: Amphineura. Conchifera. Orders of Con- 
chifera. Structure of a snail. Structure of a clam. Structure of 

of a squid 275 

Phylum 9. Adelochorda 284 



Phylum io. Urochorda. Classes of tunicates 285 

Phylum ii. Acrania 286 

Phylum 12. Vertebrata: Cyclostorhata. Pisces. Orders of fishes. 

Amphibia. Orders of amphibia. Reptilia. Orders of reptiles. 

Aves. Orders of birds. Mammalia. Orders of mammals . . 287 


Spontaneous generation. Continuity of the living substance. Struc- 
ture of protoplasm. The nucleus. Chemical structure of 
protoplasm. Function of cytoplasm and nucleus. Cell division. 
Number of chromosomes. Nucleoli. Centrosomes. Spindle 
fibres. Resting nucleus. Conjugation. Fertilization. Matur- 
ation. Conjugation in protozoa. Fertilization stimulus. Cleav- 
age. The blastula. The gastrula. The medullary plate. The 
notochord. The mesoderm. Other types of cleavage. Origin 
of the tissues. Indirect development. Differentiation of germ- 
inal and somatic tissues. Division of labor and differentiation. 
Regeneration. Mechanics of growth. Progressive and regressive 
development. Sexual dimorphism. Polymorphism. Alterna- 
tion of generations. Life habits depending on food. Parasitism. 
Protozoa as parasites. Bacteria as parasites. Immunity . . . 307 

Species. Variation. Heredity. Mendel's law. Physical basis of 
heredity. Number of species. Origin of species. The Taxo- 
nomic Series. The Phylogenetic Series. The Ontogenetic Series. 
The struggle for existence. Natural Selection. Animals and 
plants under domestication. Geographical distribution. 382 

Adaptations: Pollination. Care of young. Sexual dimorphism. 
Sexual selection. Welfare of the individual and of the species. 
Animal coloration. Protective resemblance. Feigning. Mim- 
icry. Color changes. Luminescence. Electrical organs. In- 
stinct. Intelligence 40*; 

Index ( 



1. Biology is the science which treats of living things, or of 
objects having life. In the broad application of the term, as 
used here, Biology includes a number of more special sciences. 
Morphology, Anatomy and Histology treat of form and struc- 
ture. On the other hand, Physiology deals primarily with the 
function of organs. In Embryology it is the development of 
the individual, especially during the earlier stages, that is kept 
chiefly in view. Paleontology treats only of fossils, that is, those 
types of living things which existed at some earlier period in 
the world's history but which have now become extinct. 

2. Even with such a sub-division of the subject we have 
left special sciences which cover such a broad field that they 
become unwieldly. In Botany and Zoology the subject is 
divided on the basis of the kinds of living things considered, 
the former being the biology of plants, the latter the biology 
of animals. Still further sub-division leads to Cryptogamic 
Botany, Phanerogamic Botany, Invertebrate Zoology and 
Vertebrate Zoology. Bacteriology, Entomology (insect zool- 
ogy), Ornithology (bird zoology), and still other more narrowly 
restricted branches of biology are recognized. The very ex- 
tensive study of man has given rise to a number of biological 
sciences dealing only with this single genus; viz., Human 
Anatomy, Human Physiology, Human Embryology, Anthro- 
pology (dealing with the comparative anatomy of the various 


races of men), and Ethnology (dealing with manners, customs, 
language and other activities of the races of men). 

3. Biology deals with objects, that is, with the concrete, 
but its chief interest and value lies not in mere description or 
enumeration so much as in the generalizations, which may be 
made from accumulated facts. A single observation, or 
repeated observation of a single individual seldom justifies a 
general conclusion, but by the comparison of numerous examples 
one is enabled to distinguish the accidental and trivial from the 
general and significant. Therefore, the method of study by 
comparison is for the biologist of special importance. 

(For the individual, and especially for the average college 
student who devotes a comparatively short time to the subject, 
the study of many individual examples is impossible and, there- 
fore, in practice, an abridgement of the method of study by 
comparison is adopted. This is called the method of study by 
types, by which a series of examples are compared, each example 
being representative or typical of a considerable group. That 
the examples selected are typical, rests, of course, on the 
observation of previous students or investigators. By this 
method the student may in a comparatively brief time extend 
his studies over a large field.) 

4. It is usually not difficult to distinguish a living thing from 
one not living, but to state formally what are the attributes 
of the living is not so simple a matter. On close analysis 
living things are found to be complex in structure, being 
composed of many parts, called organs, which differ in struc- 
ture and in function. For this reason a living thing is called 
an organism, and is said to be organized. 

5. Crystals are more like organisms than any other non- 
living thing, and a comparison of organisms and crystals will 
serve to indicate the most essential characteristics of living 

6. One of the most prominent characteristics of objects that 


have life is growth: crystals also grow, but not in the same 
way. The method of growth in crystals is by accretion, i. e., 
by addition of substance to the outside; but in the growth of 
living organisms the added substance is taken up into the 
interior of the body, i. e., by intussusception. Moreover, in 
crystals the chemical nature of the substance added is not 
altered, while in the case of organisms the substance added 
undergoes a series of chemical changes before it is finally 
really part of the growing body. This process of transforming 
food material is called assimilation and is wholly wanting in 

7. Another prominent characteristic of organisms is the 
definiteness of the shapes which they assume. Each plant or 
animal is as much like every other individual of the same 
kind as if they were all made after the same pattern or cast 
in the same mold. Crystals are bounded by plane surfaces 
which meet at definite angles but within this limitation the 
shape may vary indefinitely. 

8. The size of organisms is limited. The individual grows 
more or less rapidly until it reaches a certain size after which 
growth almost or wholly ceases. The size of crystals has no 
definite limitations. 

9. Crystals may be formed under favorable circumstances 
wherever the substances of which they are composed are found. 
But we have no knowledge that a living thing is ever formed 
under any conditions except by development from what we 
may call a germ, which came from some pre-existing living 
thing, and which differs from the mature organism chiefly in 
being smaller and simpler in structure. The crystal may be 
reduced to its constituent elements which will again unite 
under the proper conditions to form a new crystal, whereas if the 
organism is similarly reduced, its elements will under no condi- 
tions recombine to produce a new organism. 

10. Crystals may exist indefinitely, but the life of the in- 


dividual organism is limited. It comes to an end by death and 
subsequent decay, by division, or by fusion of its body with 
another similar body. In either case the living individual, as 
such, ceases to exist. 

11. Crystals are inert, while organisms possess to some degree 
the power of movement in response to an external stimulus. 

12. Thus we have the material world made up of lifeless, or 
inorganic bodies, and living, or organic bodies. The following 
table exhibits in parallel columns the similarities and differences 
of the two classes of bodies: 


1. Are unorganized. i. Are organized. 

2. Grow by accretion (so 2. Grow by assimilation and 
also hailstones, concretions, intussusception, 

3. Have indefinite shape and 3. Have definite shape and 
plane surfaces. curved surfaces. 

4. Size not limited. 4. Size limited. 

5. Generate spontaneously. 5. Develop from a germ. 

6. May exist indefinitely. 6. Have a limited life period. 

7. Are inert. 7. Have power of motion. 


13. All the activities of an organism, by which it is distin- 
guished from inorganic bodies, are the activities of the living 
substance, which is called protoplasm. But not all of the sub- 
stance of an organism is protoplasm. Besides the protoplasm 
there is usually more or less inert substance which was formed 
by the protoplasm, but which does not of itself possess life. 
Of such substances are the hard parts of bones and the super- 
ficial layers of the skin, the corky layers of bark and the hard 
fibres of wood, etc. This inert substance may be wholly want- 
ing, or it may constitute the larger part of the body of the 


14. Besides the protoplasm and the inert substances formed 
by it, there are in many cases foreign substances to be found 
within the various organs of an organism; 

such are, for example, the pebbles found in 
the gizzard of certain birds,, the particles of 
sand found in the antennal organ of the 
crayfish, or the sand used by many minute 
animals in forming a skeletal shell or test. 

15. Protoplasm is jelly-like in consis- 
tency, and transparent, but not perfectly 
homogeneous. Under the microscope it 
is seen to consist of an infinitely large 
number of minute particles of various sizes 
and of different optical and chemical char- 
acteristics. Chemical analysis shows that 
it is highly complex; consisting largely of 
carbon, oxygen, hydrogen and nitrogen, 
with small quantities of sulphur, and occa- 
sionally phosphorous, manganese, magnesium, calcium, sodium, 
and chlorine. It is regarded as a more or less definite aggre- 
gation of a large number of chemi- 
cally complex bodies. 

1 6. In the smaller, microscopic 
organisms the protoplasm may 
usually be observed to consist of two 
parts, nucleoplasm and cytoplasm. 
The nucleoplasm, in the form of a 
round or oval body, the nucleus, 
occupies the centre of the mass and 
is surrounded by the cytoplasm. 
The nucleus and the surrounding 
cytoplasm together are called a cell. 

In larger organisms there are a large number of nuclei quite 
regularly distributed throughout the protoplasm. There are 

FIG. i. The test of 
a protozoan, Difflugia, 
composed of minute 
grains of sand cemented 

FIG. 2. Diagram of the cell. 
For details see Fig. 179. 


then as many cells as there are nuclei, and, frequently, each 
cell is marked off from its neighbors by a wall of inert substance 
secreted by the protoplasm. The character of this wall varies, 
not only with the kind of organism, but also with the organ in 
which it is found. In the central part of a tree trunk the thick 
cell walls form the firm substance of wood; similarly, near the 
surface they form the corky layers of the bark. 

17. What has been said in the preceding paragraphs has 
a general application to all organisms, but the obvious grouping 
of living things into two kingdoms the vegetable and the 
animal is based on certain distinctive peculiarities which are 
of such far-reaching significance and which separate the more 
familiar forms of the two groups so precisely that it will be con- 
venient to study each group separately. Plants are on the 
whole much simpler than animals and therefore better adapted 
for introductory study. 



I. Light Relation of Leaves 

An erect stem with opposite leaves (Coleus). 

a. Horizontal view showing stem (nodes and internodes) and leaves. 

b. Make a diagram showing arrangement of leaves as seen from 
above. How many vertical ranks are there? 

An erect stem with whorled leaves (Galium). Horizontal view as 

in la. 

An erect stem with alternate leaves (Quercus). 

a. Horizontal view as in za. 

b. Diagram as in ib. 

Leaf Rosette (Plantago) as in ib. 

Horizontal stem with alternate leaves (Castanea). Vertical view 

showing carefully how each leaf is connected with the stem. 

Horizontal stem with alternate leaves (Tropaeolum) . Horizontal 

view. Show relation of leaves with stem as in 5. 

Leaf Mosaic (Ampelopsis Veitchii) seen from the direction of the 

midday sun. 

Inverted stems. 

a. Wistaria. Relation of leaves to stem (note the pulvinus). 

b. Salix Babylonica. Relation of leaves to stem. 

H. Form of the Plant as Related to Light 

a. Form of a tree in an open space (Pinus). 

b. Form of same kind of tree growing in a thicket. Note difference 
in lower branches in a and b and explain. 

An excurrent type of stem (Populus). 
A deliquescent type of stem (Ulmus). 



HI. Phyllotaxy 

12. Recall the opposite and whorled types of phyllotaxy. 

13. a. Arrangement of leaves in grasses (Zea Mays). 

b. Arrangement of leaves in sedges (Carex). 

c. Arrangement of leaves in sour wood (Oxydendron), oak (Quer- 
cus), etc. 

d. Arrangement of leaves in mullein (Verbascum), goldenrod 
(Solidago), Easter lily (Lilium Harrisii). 

e. Arrangement of leaves (scales) in pine (Pinus). 

IV. Morphology of the Leaf 

14. Structure of a typical simple leaf (Pyrus). Identify: blade, petiole, 
stipules and (in the blade) midrib, veins, and veinlets. 

15. Types of venation: Reticulate a. Pinnate (Castanea), b. Palmate 
(Ampelopsis Veitchii). Parallel c. Basal (Convallaria), d. Costal 

16. Form of the margin of simple leaves. Find examples among the 
leaves already studied of a. Entire, b. Serrate, c. Lobed. In 
addition draw one that is d. Parted (Ricinus), and e. Divided 

17. Compound leaves: a. Pinnately compound (Robinia) note rachis 
and leaflets, b. Palmately compound (Parthenocissus). 

1 8. Structure of the blade, a. With the edge of the scalpel strip off the 
thin membrane (epidermis) covering the upper and lower surfaces 
of the blade (Caladium). Study the epidermis with a hand lens and 
with a needle determine the texture, b. With the lens study both 
surfaces of the green part of the blade (mesophyll) where the epi- 
dermis has been removed. The denser portion is palisade mesophyll, 
the other, spongy mesophyll. Determine texture with the needle. 

c. Scrape out some of the mesophyll and soak it for a time in alcohol 
in a test-tube. Note the result. The green matter is chlorophyll. 

d. Can you find pores (stomata) in the epidermis? 

V. Stems and Roots 

19. Study a cross section of a stem (Paulo wnia) (Sambucus). Determine 
the texture of the wood, pith and bark. 

20. Study a similar stem in longitudinal section in the region of the 
node, as in 19. 

21. Compare the root of a similar plant with the stem, as in 19 and 20. 



A dicotyledonous seed (Phaseolus). 

a. Draw two views of a bean seed to show the general form, the 
hilum, the micropyle and the chalaza. 

b. From a seed that has been soaked in water remove the seed 
coats (testa and tegmen). Study the coats. 

c. Draw the embryo, showing the cotyledon and caulicle. 

d. Separate the cotyledons and draw to show the cotyledons, the 
plumule and the caulicle. 

Other dicotyledonous seeds: 

a. Study a seed of Pisum as in 220, b, c, and d. 

b. Compare seeds of Trifolium and Raphanus (or Brassica) with 
those of Phaseolus and Pisum. 

c. Study a seed of Cucurbita as in 220, b, c, and d. 
A monocotyledonous seed (Zea Mays). 

a. Draw that side of a grain of corn which shows the embryo. 

b. Remove the seed coats from a specimen which has been softened 
in water and find the embryo embedded in the mealy endo- 
sperm. Study the seed coats, but do not try to homologize 
with those of the bean. They are more complex. 

c. With a sliding cut make a longitudinal section of a seed through 
the shorter diameter so as to exactly halve the embryo. Draw 
the section and identify cotyledon, plumule and caulicle. 

d. Cut another transversely at three points, so as to divide the 
embryo into quarters. Draw the three sections. 

e. Compare a seed of Triticum with that of Zea. 

A seed of a Gymnosperm (Pinus). Study a pine nut noting the 
character of the seed coat, endosperm and embryo. 

VII. Development of the Seedling 

Where in a germinating bean does evidence of growth first appear? 

Where and how does the developing part first emerge from the 

seed coats? Compare plumule and caulicle with regard to rate of 

development during the first week of growth. 

Compare pea, squash and corn with bean in regard to each point 

mentioned in 26. 

How does each of the four kinds of seedlings emerge from the soil? 


29. Compare the cotyledons of the seeds in regard to their behavior 
during the early stages of development. 

30. Why does the primary root grow downward? To be determined 
by experiment i. 

Experiment i. Spread a piece of moist white filter paper in the bottom 
of a shallow plate or pan. On this set a bottle about three inches high 
with a cork projecting from the neck. Fasten several pea seeds to the 
cork with long pins in such a way that they will be suspended in mid-air 
at least. an inch from the cork. The pins may be thrust through the 
seed coats or the cotyledons but the caulicle and plumule must not be 
injured. The seeds should be fastened so that the caulicle is directed 
downward in one case, upward in another and horizontally in another. 
Cover the whole with a bell jar and note the direction taken by the caulicle 
when it germinates. Interpret the result. See paragraph 49. 

31. Why does the stem grow upward? To be determined by experi- 
ment 2. 

Experiment 2. Seal up the hole in the bottom of a four inch flower pot. 
Fill the pot with earth or sand and plant some pea seeds about an inch 
below the surface. Moisten the soil and then cover the pot with wire 
mosquito netting so that the earth will not fall out when the pot is in- 
verted. Invert the pot and support it on an empty glass tumbler and 
cover the whole with a tall bell jar. The tumbler should be set in a shallow 
dish or pan on a piece of wet white filter paper. After a period of about 
ten days, carefully raise the pot, allowing the soil to fall away and expose 
the seedlings. Interpret the result. See paragraph 50. 

32. What changes occur in the cotyledons and what is their ultimate 
fate? What is the function of the cotyledons? 

33. The young plant a. bean, b. pea, c. squash, d. corn. 

34. Reviewing development as if in a moving picture, describe the devel- 
opment of the plumule. 

35. As in 34 describe the development of the caulicle. 

36. The hypocotyl is that part of the stem which develops from the 
caulicle. Compare the hypocotyl of bean, pea, squash and corn. 

37. Root hairs (wheat). Study under glass cover. 

38. Draw root system of well developed bean. Note, primary, or tap 
root, and, secondary, or lateral roots. 

39. The cotyledons are the first leaves. Are the leaves that develop 
next like those of the mature plant? Are the third? Fourth? 
Compare bean, pea and squash. 

40. Where do the branches appear? See also section viii. 


Vm. The Mature Plant 

The root (Quercus alba) in cross section. Draw x$. Study the 
details carefully with hand lens. Is there a central pith (medulla) ? 
Medullary rays? Vessels or tracheae in the wood. How arranged? 
Components of the bark? 

With scalpel and needles determine the texture of the various parts 
of the root. 

Are the vessels true tubes ? Can you blow through them ? (Exp. 3 .) 
Longitudinal section of root tips (microscope, prepared slide) . Note 
the arrangement of the cells, and the root cap. 
The stem (Quercus alba) (Aristolochia) in cross section. Draw X4. 
a. One year old stem. b. Two year old stem. c. Three year 
old stem. Note annual rings, medullary rays, epidermis, cork, 
chlorophyll. Compare in the three sections, the pith, the wood 
and the bark. What changes occur as the stem grows older? 
A branch at least three years old. Can you determine from surface 
appearance the limits of the i, 2 and 3 year old parts? How? Lo- 
cate the limit of the last season's growth (the twig). 
Draw the twig (white oak) showing the scale leaf scars, foliage leaf 
scars, terminal and lateral buds. What is the function of these 
buds? (see 48). Are there any branches? (see 48). 
Study the growth of the preceding season. Are there any branches? 
How old are they? Where do they occur? What determines the 
position of a branch on a stem? 

How old is the basal portion of your branch? Determine by surface 

Dissect a bud (Hicoria or ^Esculus). Note the character of the bud 
scales and their arrangement. What is their funtion? What do 
you find in the center of the bud? 

Structure of the stem (white oak). Study: a. cross, b. longitudinal 
radial and c. longitudinal tangential sections of a block at least 
eight years old. Note epidermis, cork, chlorophyll, other tissues 
of the bark. What is the "grain" of wood? What are the flakes 
in quartered oak? Why must the wood be "quartered?" 
Compare the vessels of the root and stem. Compare roots and 
stems of the same diameter with regard to rigidity. 
Structure of the stem (corn), a. Study cross section of the corn 
stem noting carefully the arrangement of the pith and vascular 
bundles, b. Split the stem and draw. Are the vascular bundles 
continuous? c. Set a section of stem in red ink (Exp. 4). Does 


the ink rise in the vascular bundles? d. A prepared slide under 
the hand lens showing the vascular bundles and the vessels. Draw 
one bundle X25. 

54. The leaf: a. Surface view of the epidermis of a leaf showing the sto- 
mata (prepared slide, microscope), b. Cross section of the leaf 
(prepared slide, microscope). Identify the layers found in 18. 

Modified Roots 

55. Fibrous roots (grasses). 

56. Enlarged (storage) roots. 

(a) Enlarged tap-root (turnip). 

(b) Enlarged fascicled roots (Dahlia). 

(c) Enlarged lateral roots (sweet potato). 

57. Prop roots (corn). 

58. Aerial roots as holdfasts (ivy). 

Modified Stems and Branches 

(In each case note the nodes and internodes and the character of the 
leaves and buds.) 

59. Procumbent stems (periwinkle). 

60. Runner or stolon (strawberry). 

61. Underground stems: 

(a) Rootstock (Smilax). 

(b) Rhizome (Solomon's seal). 

(c) Tuber (Irish potato). 

(d) Corm ( Jack-in- the-pulpit). 

62. Climbing stems: 

(a) Twining stems (morning glory) . 

(b) Climbing by spiral tendrils (grape). 

(c) Climbing by adhesive tendrils (Virginia creeper). 

(d) Climbing by aerial roots (ivy). 

63. "Stemless" plants: 

(a) Study a plant of salsify. Is there a stem? Where does the root 
begin? Note the leaf scars. 

(b) Make a longitudinal section of root and stem. Note distribution 
of pith and vascular bundles. 

(c) Make cross sections of the stem and the root. Note again as in b. 

(d) Compare a beet or turnip with the salsify as in a, b and c. 


Storage stems (cactus). What is stored? Note the condition of 
the leaves. Where is the chlorophyll? 

Cladophylls (Myrsiphyllum). What are the small scalelike struc- 
tures borne by the stems ? What are the leaf-like organs (cladophylls) 
borne in the axils of the scales? 

(a) What are the thorns on the black locust? 

(b) What are the thorns on the holly and barberry? 

(c) What are the thorns on the honey locust? 

Tendrils: Compare the tendrils of the grape and Virginia creeper. 
Why are they branches? Note how the grape tendril is coiled. 

Modified Leaves 

Scale leaves. Recall various types already studied. 

Tendrils. Compare leaves of the pea, vetch and vetchling. What 

are the tendrils in each? 

Thorns. See paragraph 66, a and b. 

Storage leaves: 

(a) Compare leaves of Portulaca and the houseleek. 

(b) Make a longitudinal section of an onion. What are the scales? 
(Where is the stem?) 


(a) The sundew (Drosera). Note how the hairs on the leaves react 
to contact with a gnat or other small insect. 

(b) The pitcher plant (Sarracenia) . What devices are employed for 
catching insects? 

(c) The Venus flytrap (Dionaea). 

X. Flowers 

A simple type of regular flower (Oxalis). 

(a) Make a diagram of the flower as it would appear in longitudinal 

(b) Make a "plan" diagram to show how the parts are arranged 
around the center. 

(c) Draw one member of each cycle. For terms see text. 

(d) Make a cross section of the ovary. 

A simple type of irregular flower (Swainsona). Study as in 73. 
Study other more modified types of flowers such as Primula, etc. 


XI. Fruits 

76. Simple fruits; dry, dehiscent. 

(a) A follicle (milkweed). 

(b) A legume (bean). 

(c) A pod (Yucca). 

77. Simple fruits; dry, indehiscent. 

(a) A samara (maple). 

(b) An achene (sunflower). 

(c) A caryopsis (wheat). 

(d) A nut (oak). 

78. Simple fruits, fleshy: 

(a) A drupe (plum). 

(b) A pome (apple). 

(c) A berry (cranberry, persimmon). 

79. Aggregate fruits (Magnolia). 

80. Multiple fruits (pineapple). 

XII. Classes of Plants 

For distinguishing characters see pages 71 ff. and 101 ff. 

81. Dicotyledons. Make a list of at least ten common dicotyledonous 

82. Monocotyledons. Make a list of at least ten common mono- 
cotyledonous plants. 

83. Gymnospermae. Make a list of all the kinds of Gymnosperms grow- 
ing in your vicinity. 

84. Pteridophytes. Dig up a fern with all the roots and wash away the 
soil. Draw to show roots, rootstock and leaves. Study the under 
surface of a fruiting frond with the lens. 

85. Bryophytes. Musci Collect a number of kinds of moss. Find 
plants with and without a spore capsule but otherwise alike. Draw 
both kinds. 

86. Bryophytes. Hepaticae Collect and study liverworts as in 85. 

87. Lichenes. Collect several kinds of lichens. 

88. Algae. Collect several kinds of algae. 

89. Fungi. Collect one or more kinds of each of the following fungi: 

(a) Mushrooms, toadstools, puffballs, rusts and smuts. 

(b) Mildews, blue and green molds and black fungi. 

(c) Water molds and black molds. 



1 8. Plants are usually green. This is so commonly true 
that perhaps the most general idea associated with the term 
plant is that of the green color. There are, however, many 
plants that are not green, as, for example, the " dodder" and 
"indian pipe." But these plants are also exceptional in other 
ways. The red and brown sea weeds and plants like the 
coleus are apparently exceptions, but in these cases the green 
is really present, though masked by other coloring matters. 
Besides, there is a large group of organisms like toadstools 
and molds, collectively called fungi, which are not green. 
These organisms are grouped by the biologist with the plants, 
but they are evidently very different from what is commonly 
meant by " plants," and for the present we may leave them 
out of consideration. So we may say that, with some excep- 
tions, those organisms which are commonly called plants, are 
green. Such uniformity of color is not found among animals 
and, therefore, it is worth while to ask, why are plants so 
uniformly green? 

19. Most plants which are normally green lose their color 
when grown in the dark. Thus grass growing beneath a stone 
is yellow; celery is blanched by covering it, and the shoots of 
potatoes sprouting in a dark bin have no trace of green. 

20. Exposure to sunlight soon produces the familiar green 
in the leaves and certain parts of the stem of such etiolated 
plants. Frequently those parts of a plant which are not nor- 
mally green become so on exposure to sunlight. This occurs 
when, for example, the tubers of an Irish potato, normally 
underground, are exposed by removal of the soil. The same 
is true of the roots of many plants. 

21. Furthermore, it will be found that no green plants will 
continue to grow in places where they can get no light; while 
on the other hand fungous growths like toadstools flourish in 


cellars, caves, hollow logs and similar dark situations where 
green plants are never found. 

22. There seems, therefore, to be a direct relation between 
the green color of plants and the sunlight, and this becomes 
still more evident when we consider the distribution of the 
green on the individual plant. In the smaller herbaceous 
plants the green may be found in all parts above ground, in 
stem and leaves alike; but in the larger perennial growths, like 
the oak tree, the green is found only in the leaves and twigs; 
possibly also on the surface of the smaller branches and between 
the ridges of dead bark on the larger limbs and the trunk itself. 
But in the dark central parts of the branches and trunk, and 
beneath the thick ridges of dead bark there is no green. It 
may be wanting entirely in the stem, but, with the rarest 
exceptions, it is always present in the leaves. 

The Leaves 

23. In connection with its color it is also important to note 
the form of the leaf. This varies through an infinite variety 
of patterns from circular to linear, but in almost every case it 
is either very small, or else very thin or very slender in propor- 
tion to its other dimensions. From this it results that the 
surface of the leaf is large in proportion to its volume, and all 
of its substance lies near the surface, that is, exposed to the 
light. In other words, the form of the leaf is such as to give a 
maximum exposure of its substance to the light. As a general 
characteristic of leaves this one of form is second in importance 
only to that of the green color. 

24. A typical leaf consists of three parts: (i) a broad, thin 
portion the blade, (2) a narrow rounded or angular stem 
the petiole, and (3) at the base of the petiole a pair of wing-like 
appendages the stipules. Stipules vary greatly. They are 
usually more or less leaf-like, but may be reduced to mere 


rudimentary structures or may even be entirely wanting. The 
leaf may also be without a petiole, in which case the blade is 
directly connected with the stem or branch of the plant and is 
then said to be sessile. 

25. The petiole is stiff enough to support the blade and yet 
is flexible and elastic. It is composed largely of fibrous woody 
tissue, which extends on from the petiole into the blade, where 
it is so disposed as to constitute a framework upon which the 
more delicate tissues of the blade are supported. This frame- 
work is made up of one or a few large ribs which by branching 
give rise to numerous smaller veins and veinlets. 

26. The veins may be arranged in one of two ways; they 
either lie parallel with one another and extend from the base 
of the blade to its tip or from the single large midrib to the 
edge of the blade, or else they unite with each other in such a 
way as to form a network. Leaves having the former arrange- 
ment of the veins are said to be parallel veined, while those pre- 
senting the latter condition are termed netted veined. Netted 
veined leaves may further be characterized as feather veined 
if there is only a single large midrib from which the principal 
veins branch on either side, or palmately veined when there are 
several ribs spreading in a fan shaped order from the base of 
the blade. 

27. The upper and lower surfaces of the leaf blade are formed 
by thin, transparent, but rather tough, membranes, the epider- 
mis, which may be stripped off. Between the two layers of 
epidermis lies the green mesophyll, which next the upper epider- 
mis forms a rather firm tissue, but on the lower side is more 
spongy in texture. 

28. In outline the leaf blade is extremely variable. All forms 
from circular to narrow ribbon-like or even thread-like are met 
with, and the margin varies from a continuous line or unbroken 
curve to conditions which may be described as toothed, lobed, 
cleft or divided, as the case may be. A divided leaf is one in 


which the indentations of the margin extend completely to the 
midrib, thus producing a double series of leaflets ranged along 
a common midrib. 

29. When the divisions of the blade are all distinct so that 
each resembles a miniature leaf, the leaf is said to be compound. 
The divisions are then called leaflets and the common leaf- 
stalk is the rachis. 


30. Leaves are usually arranged on the stem in a definite 
order. On a vertical shoot there are two or more vertical ranks 
of leaves. When there are two leaves at the same level they 
are opposite, and each pair crosses the pair above or below at 
right angles, making four vertical ranks of leaves. Sometimes 
there are three or more leaves at the same level, forming a 
whorl. If there is only one leaf at each level the leaves are said 
to alternate. In this case every 2nd, 3rd, 5th, 8th or i3th, 
etc., leaf, as the case may be, is in the same rank and there will 
be 2, 3, 5, 8, or 13, etc., ranks respectively. This order is in 
reality a spiral one, for if a line is drawn from one leaf to the next 
higher one in the nearest direction and continued in this way it 
will describe a spiral around the stem. This methodical ar- 
rangement of the leaves evidently gives each leaf a maximum 
of elbow room with respect to its fellows, and tends to equalize 
the conditions of light and shade. 

31. The number of leaves which may receive sufficient light 
exposure on a stem of a given length depends on (i) the size of 
the leaf a few large ones will shade each other as much as a 
large number of small ones, (2) the shape of the blade long, 
narrow leaves or finely divided ones may be set more closely 
than broad and entire leaves, (3) the length of the petiole- 
other things being equal a long petiole will give the leaves 
more room than a short one and consequently long petioles 


are usually associated with broad leaves. From this it follows 
that there is a correlation between the number of ranks of 
leaves on the stem and the distance between leaf levels on the 
one hand, and the form and size of the leaf on the other. 
Plants which normally grow in tussocks, i. e., many stems in a 
cluster, form a natural exception to the above rule, for in this 
case the crowding of the stems reduces the number of leaves 
possible on each stem. 

32. On horizontal branches the leaves are attached to the 
stem in precisely the same order as on vertical stems, but the 
blades of the leaves are in many cases brought round into the 
horizontal plane by a twisting and bending of the petiole. 

33. Still other devices are employed for securing equal and 
sufficient illumination of the leaves, but whatever the means 
employed all tend toward the same result, viz., a maximum 
exposure of green tissue to the light. 

The Stem 

34. From what has gone before, it is evident that one of 
the functions of the stem of the plant is to hold up the leaves 
to the light. This function may be performed in various ways, 
and much of the character of the stem and its branches depends 
upon how this function is performed. Among the low herba- 
ceous forms the adaptation of the stem to this function is simple 
enough and nothing further need be said here, except to note 
that where the plants are crowded the stems are usually less 
branched, and more slender than where they grow singly. 
This is also true of trees, and the cause of it may be discovered 
by the comparison of a few examples. 

35. A tree growing in an open space tends to have a relatively 
short and thick trunk, with large, spreading branches near the 
ground. One growing close by the side of another in an open 
field will have the large branches only on the side away from 



the neighboring tree. If the tree is closely surrounded by others 
of approximately the same age, as in a forest, the trunk will be 
taller in proportion to its diameter and there will be no large 
limbs near the ground. These facts show clearly that the larger 
branches develop only where they can reach the light, whereas 

FIG. 3. Three oak trees 'in a group, showing the effect of one tree on 
another with regard to the development of the branches. In the middle tree 
the branches extend toward, and away from, the observer. 

those twigs which appear from time to time in shaded situ- 
ations fail to develop into large branches because of the lack of 
light, and, after a few seasons' struggle against adverse circum- 
stances, ultimately die and fall away. 

36. The stem of erect plants is usually a cylinder of woody 


tissue. Woody tissue possesses in a large degree both rigidity 
and elasticity, while the cylindrical form is of all forms the one 
giving greatest rigidity. The stem cylinder is often hollow, 
but usually the axis is occupied by a core of spongy tissue the 
pith. This by itself would be of little value as a supporting 
structure and yet in young shoots it probably adds greatly to 
the strength of the stem by preventing buckling of the cylinder. 
In far the greater number of the plants there is also a cylinder 
of bark which surrounds the woody part. This bark has a 
double function. It adds greatly to the elasticity of the stem 
through the layer of fibers the bast which lies directly over 
the wood and which possesses great tensile strength. The 
other function of the bark that of protection is subserved 
by its outer layers which consist either of a smooth and tough 
epidermis or thick layers of corky tissue, both of which are 
highly resistant to mechanical injury. 

The Roots 

37. The stem of the plant is firmly anchored in the soil by 
the roots. Continuing downward from the base of the stem 
there is often a short, rapidly tapering tap root, while other 
and longer roots pass out radially and usually at a small angle 
downward. These master roots branch repeatedly, giving rise 
ultimately to a vast number of minute rootlets which interlace 
and penetrate the soil in all directions and through a space of 
considerable radius. The tap root is often insignificant and 
the plant is held erect by the combined bracing and guying 
action of the lateral roots. Only the larger roots where they 
unite with the stem possess any great degree of rigidity. The 
more remote parts of the root system have little rigidity or 
elasticity in comparison with the stem and branches. They 
are, on the other hand, quite flexible and tough and capable of 
resisting a considerable pull longitudinally. 


38. In many cases there are a large number of fibrous roots 
which spring directly from the base of the stem instead of a 
few larger roots. Such fibrous root systems occur principally 
on low herbaceous plants which do not require an especially 
strong supporting system. 

39. The structure of the root, in the main, resembles that of 
the stem in that there is a central woody axis and an outer 
bark; but there is no pithy core. The woody portion is not as 
firm as that of the stem, but it is of ten very tough. The outer, 
dead, protective layer of the bark of roots is also relatively 

40. In the preceding paragraphs some of the most evident 
characteristics of ordinary plants have been noted for the 
purpose of showing that the form and structure of plants is in 
every case an adaptation to a certain end, and that even the 
peculiarities of each kind of plant are special adaptations to 
special ends. In continuing our studies we shall constantly 
keep in mind this idea of adaptation to functions, that is to 
say, at every point we shall seek to answer the question why? 

41. In the following paragraphs we shall take up for con- 
sideration, in turn, the seed, the developing seedling and the 
mature plant, studying in each case the structure and functions 
of the principal sets of organs, so that at the end we may have 
a fairly comprehensive idea of the life-history of a plant. 


42. Seeds present a remarkable diversity of form and struc- 
ture, but there are usually two distinct sets of organs to be 
recognized. The first of these are the seed coats, evidently 
organs of protection, often consisting of an outer firm layer, 
the testa, and an inner membraneous layer, the tegmen. How- 
ever, the seed coats may be variously modified and cannot 
be generally characterized. 


43. The other set of organs is the essential part of the seed 
and constitutes the germ or embryo. In the largest of the three 
grand divisions of seed-bearing plants, the Dicotyledons, the 
embryo consists usually of two symmetrical parts, the cotyle- 
dons, which are connected by a third part the caulicle. At 
the end of the caulicle between the cotyledons there may also 
be a minute structure, the plumule, which when well developed 
shows clearly the outlines of one or more leaves in miniature. 

44. In another division of the seed-bearing plants there is 
only one cotyledon, and hence the name applied to the group 
is Monocotyledons. 

45. Besides the embryo there is frequently contained within 
the seed coat a mass of food material for the use of the develop- 
ing embryo. This food material, called endosperm or peri- 
sperm, depending on the position it occupies in the seed, may 
consist either of starch, proteid, oil, or cellulose, or a combina- 
tion of two or more of these food principles. 


46. The conditions necessary for the germination of seeds 
are: First, a favorable temperature which might be designated 
as warmth. The range and limits of this favorable temperature 
are not sharply denned and may vary with the kind of seed. 
Cold, a temperature below the limit within which germination 
takes place, indefinitely retards development, though it does 
not necessarily destroy the vitality of the germ if the seeds are 
dry, while on the other hand, any considerable increase of 
temperature above that of germination destroys all power of 
further development. 

47. A second condition of germination is moisture. This 
softens the seed coats, thus permitting the embryo to expand, 
and also supplies the water which is everywhere necessary to 


48. That oxygen is necessary to germination may be demon- 
strated by experiment either by placing the seeds of aquatic 
plants in water from which the air has previously been expelled 
by boiling, or by placing seeds in a vessel containing an atmos- 
phere deprived of its oxygen. 

49. Under the conditions just enumerated the embryo swells 
through the absorption of water, the seed coats burst and the 
caulicle grows out and down into the soil, the terminal part of 
it going to form the primary root. It will be noted here that 
the direction of growth, not only of the root, but also of the other 
growing parts of the plant, is very definite and that the deter- 
mining cause of it must be sought in some external agency. 
By suitably conducted experiments we find that gravity acts 
upon the primary root as a stimulus, in response to which it 
grows downward. This response is known as positive geotrop- 
ism. If the influence of gravity be eliminated the root will turn 
toward the source of moisture this is positive hydrotropism. 

50. The plumule also responds to external stimuli, but in 
a different way. It turns away from the earth, being negatively 
geotropic, and grows toward the light positively heliotropic. 
(The student should note that unless light actually impinges 
on the seedling it can have no influence in determining the 
direction of growth. Hence, if the seed is growing in the 
dark the direction of growth must be determined by some 
stimulus other than light. In this connection analyze care- 
fully the results of experiments i and 2 under paragraph 30, 
and 31, page 10.) 

The Seedling 

51. When the primary root has penetrated the soil some 
distance, lateral branches begin to appear on all sides of it 
at some distance above its tip. These branches are not posi- 
tively geotropic, since they grow in an almost horizontal direc- 
tion diageotropism with perhaps a slight tendency down- 


ward. With the appearance of the lateral rootlets there can, 
of course, be no further elongation of that part of the radicle 
or tap root from which they spring, since this would result only 
in the destruction of the branch roots or a doubling of the tap 
root. Observation of a marked primary root shows, in fact, 
that elongation takes place only near the tip. The subsequent 
development of the root system is simple enough. The main 
branches increase in diameter, and, as they push out farther 
into the soil, give off numerous smaller branches. Successive 
branching in this way finally produces a system which ends 
in innumerable minute rootlets. 

52. With the development of the lateral roots the seedling 
becomes firmly anchored in the soil. This is a necessary 
preliminary in many plants to the first steps in the development 
of the stem. In some cases the conical plumule pushes upward 
through the soil as the radicle grows downward, without moving 
the cotyledons. In other cases the cotyledons are forced up 
through the soil before the plumule has undergone any con- 
siderable development. This is accomplished by the elongation 
of that part of the seedling called the hypocotyl which lies 
between the cotyledons and the first lateral roots. With one 
end fixed by its root anchorage, the elongating hypocotyl 
carries the cotyledons upward in the direction of least soil 
resistance. During this process the seed coats are" stripped off, 
and, as soon as the cotyledons appear above ground, the 
plumule is free to continue its development. 

53. The several functions of the cotyledons now become 
evident. In those cases where they remain in the soil they are 
either greatly swollen by the reserve food contained in the cotyle- 
dons themselves, or else they are embedded in a large store of 
endosperm or perisperm. In either case they nourish the grow- 
ing embryo from the stored food supply. The plumule of such 
seeds is a conical shaft, well adapted to bore its way through 
the ground. 


54. In those cases in which the cotyledons appear above the 
ground they serve to protect the delicate plumule while the 
vigorous hypocotyl is pushing it up through the earth. If the 
cotyledons in this case are greatly thickened they are likely to 
become shriveled as they give up their food to the seedling, 
and finally they may fall off. Again they may become green 
and serve for a time the functions of leaves. Very often they 
are clearly leaflike at the beginning and remain for some time 
as the first pair of leaves. In every case the cotyledons nourish 
the seedling, either from endosperm or perisperm, or from the 
food contained within their own tissues, until green leaves are 
developed by transformation of the cotyledons themselves or 
by the development of the first leaves by the plumule. 

55. The plumule is the last of the embryonic parts to begin 
its development. From it arise practically all the above- 
ground parts of the plant, i. e., stem and leaves. In the embryo 
it is essentially a bud, and, as it develops, one segment of the 
stem after another appears and leaf after leaf unfolds until we 
have the fully formed plant. The region of development, i. e., 
the formation of new parts, in the plumule is at the apex of 
its axis in the center of the bud, but after the parts have been 
formed and unfolded they continue to expand for some time. 
From this primary bud, which is first called the plumule, but 
later on is known as the terminal bud, is developed, directly, 
the main axis or stem of the plant with its leaves. 

56. Secondary axes, or branches, are developed from buds 
(axillary buds) which appear in the angles (the axil) between 
the leaves and the stem. In the case of perennial plants the 
leaves which form last, but do not unfold in the fall, and which 
are the first to unfold in the following spring, are scale-like in 
form and serve to protect the tender parts which they enfold, 
from the winter weather. 

57. In some cases accessory buds occur above or on either 
side of the axillary bud, and adventitious buds may occur on 


any part of the stem. In case a terminal bud is destroyed, and 
also under certain other conditions, the development of the 
main axis may be continued by an axillary bud. Also, if an 
axillary bud is destroyed its functions may be taken up by ac- 
cessory or adventitious buds. 

58. Since branches normally develop from axillary buds, it 
follows that branches are arranged on the stem in conformity 
with the law which governs the arrangement of the leaves on 
the stem. 

59. The terminal bud, because of its favorable position with 
respect to light exposure, and also possibly for other causes, is 
usually stronger than lateral buds, and therefore the main axis 
develops more rapidly than the branches. Many lateral buds, 
on the other hand, are in such unfavorable positions that even 
after having developed to some extent they are " choked" and 
the twig dies and falls away. Still others never develop at all. 
Thus it results that while the position of a branch on the stem 
is governed by the law of leaf arrangement, yet, because of the 
large number of buds that do not develop and of others that are 
choked out, the regularity of arrangement is seldom evident in 
the case of branches. 

The Mature Plant 

60. At the end of the growing season the foliage leaves of 
deciduous perennials fall off, leaving a scar on the twig. The 
bud scale-leaves fall away on the unfolding of the bud and also 
leave scars, which, however, are so crowded, because of the 
slight elongation of the axis, that they frequently form a con- 
tinuous ring around the stem. The scale-leaf scar can also be 
distinguished from the foliage-leaf scar by its form. The 
position of the scale-leaf scars indicates the beginning of the 
year's growth, consequently the age of a twig may be deter- 
mined by counting the successive rings of scale scars. Other 



characters, such as the color and texture of the bark and th< 
succession of branches, will also serve to determine the age o 
any particular section of the branch. 

61. If we cut across a twig of one year's growth, we find tha 
it consists of three parts, an outer bark which may be peelec 
off, a central core of soft tissue the pith and between them 


FIG. 4. Photomicrograph of a cross section of oak wood showing one year' 
growth. E, Early growth; L, late growth; m and n, large and small medullary 
rays. (From Stevens.) 

a firmer cylinder, the wood. The outer surface of the bark i 
smooth and rather tender, and covers a layer containing more 
or less green substance. The inner layers, those which are nex 
the wood, are hard and consist largely of very tough fibers 
The pith is soft and spongy in texture and contains no fibers 
The wood is also fibrous, since it can be split lengthwise of th( 


stem, but it is more compact than the fibrous tissue of the 
bark and cannot be as readily separated into strands. 

62. If twigs two and three years old are cut across we find 
that there are differences besides merely that of thickness. In 
the older stems the surface of the bark has changed color, 
become firmer and also perhaps rougher. There is less, if any, 
evidence of chlorophyll, and the bark is thicker. The pith 
shows little change, but the woody cylinder is about twice or 
three times as thick as before and is divided by concentric 
circles into annual rings of growth. Crossing these circles of 
growth at right angles are narrow radial lines of pith which 
connect the central pith core with the bark. These are the 
medullary rays. 

Composition of Plants 

63. It is evident that water constitutes a very large per 
cent, of the substance of plants. If a portion of plant tissue be 
weighed and then subjected to a moderately high temperature 
until it is thoroughly dried and then weighed again, it will be 
found to have lost from 50 to 95 per cent, of its weight. In suc- 
culent herbs the percentage of water is very great, while in 
woody tissues it is much less. A moment's thought will show 
that the water contained in plants must be absorbed chiefly by 
the roots, for plants may grow and flourish even though water 
never falls upon the stem and leaves. 

64. If after thoroughly drying vegetable tissue the tempera- 
ture be increased to just short of the point of ignition the tissue 
becomes black and there finally remains only a mass of charcoal 
(carbon), equal in weight to about 25 per cent, of the dried mass. 
During the process of charring various vapors and gases are 
driven off; among others are the vapor of water (H 2 O), carbon 
dioxide (C0 2 ), carbon monoxide (CO), marsh gas (CH 4 ) and 
other hydro-carbons. After complete ignition of the charcoal 


there is left a small residue of ash, amounting to about 5 per 
cent, or less, of the dried substance. 

65. The ash consists chiefly of the following mineral sub- 
stances, viz.: Potash, soda, lime, magnesia, phosphorous, 

FIG. 5. Experiment to determine the composition of vegetable tissue. A 
simple apparatus, consisting of test-tubes, glass tubing and cork stoppers, is 
fitted up as shown in the figure. The tube A should be of hard glass. A piece 
of dry wood (W) is then heated over a burner, at first gently, then more vigorously, 
until it is reduced to charcoal. At first water is driven off and condenses in the 
cold tube CB). Then other volatile substances pass over, some of which con- 
dense in B and others escape at C. The latter may be tested for H 2 O and CO 2 . 
The jet escaping at C may then be ignited and the flame tested for H 2 O and 
CO 2 . The liquid which has collected in the tube B, is wood vinegar and contains 
water, acetic acid, wood alcohol and tar. Test with litmus paper for acid. 
Then heat until it boils, when a blue flame may be obtained at C. This is due 
to the volatilized alcohol. 

sulphur, silica, chlorine, and manganese, which are evidently 
derived from the soil and must therefore have been absorbed 
by the roots. 

66. Plants will thrive if the water supplied to the roots con- 
tains only the above minerals and a trace of iron. But the 


largest constituents of the plant are carbon, about 45 per cent, or 
more, and oxygen, about 45 per cent, or less. Since the carbon 
is not necessarily present in the water it must be derived from 
some other source. Carbon is present in the atmosphere in 
small quantities, combined with oxygen in the form of carbon- 
dioxide (CO 2 ), and in the absence of this gas the plant will not 

FIG. 6. The preceding experiment may be performed more satisfactorily by 
substituting an iron capsule for the hard glass test-tube and connecting the 
delivery tube with a Leibig condenser. Such an arrangement is represented in 
Fig. 6. 

thrive. Consequently we must assume that the carbon is 
absorbed from the atmosphere by the stem and leaves. This 
conclusion may be verified by experiment. 

67. The oxygen taken up by the plant may be, and as a 
matter of fact is, taken up in part as free oxygen from the 
atmosphere, in part in combination with carbon as CC>2, and in 
part in combination with other elements absorbed by the roots. 


Structure and Function of the Roots 

68. The mechanism of water absorption by the roots may be 
discovered by the study of cross sections of the smaller root- 
lets. Such a section taken several centimeters from the tip, 
i. e., through the region covered by root hairs, presents three 
well marked kinds of tissues; viz., (i) a general ground tissue 
made up of rounded or polygonal cells with thin walls, (2) 
larger circular structures grouped around the axis of the root, 

which are longitudinal vessels 
in cross section, and (3) root 
hairs, which are tubular ex- 
pansions of some of the thin 
walled cells of the surface 
layer. The vessels may usu- 
ally be seen by the unaided 
eye, especially in the larger 
roots. The root hairs are 
very conspicuous and, when 
growing in a moist atmos- 
phere, stand up rigidly from 
FIG. 7. Cross section of a young root, the surface of the root as 

slender cylindrical bodies 

several millimeters in length. If they are exposed for a few 
minutes to the dry air they soon become limp, topple over and 
shrivel. This fact shows that the watery content of the hair 
is rapidly extracted by evaporation from the surface, and that, 
therefore, the cell wall of the hair is highly pervious to water. 
69. By cutting off the stem of a growing plant near the 
ground and connecting a glass tube with the stump it may be 
shown that the roots have the power, not only of absorbing 
moisture from the soil, but also of driving the sap up into the 
stem under considerable pressure. In all probability the 
force chiefly responsible for this root pressure is the osmotic 



FIG. 8. Cross section of rootlet in the region 
of the root hairs. (From Stevens.) 

action which takes place between the contents of the root hairs 

and the soil water through the cell walls of the root hairs; these 

cell walls being admirably adapted to serve as osmotic 


70. Soil water holds 

various mineral salts in 

solution in small quanti- 

ties. These are absorbed 

with the water and 

furnish the mineral con- 

stituents of the ash. At 

the same time carbonic 

acid passes out from the 

root hairs into the soil and 

by its solvent action helps to break up the mineral constitu- 

ents of the soil, thus serving at once to disintegrate the rocks 

and also increase the quantity of 
mineral salts contained in the soil water. 
7 1 . The fluids absorbed by the root 
hairs may then also be transferred 
from cell to cell by osmotic action and 
thus finally reach the tubular vessels 
which lie near the axis of the root. 
These vessels form a conducting tissue 
through which the fluids may travel 
freely, propelled by the osmotic force 

between the dotted lines is o f the thousands of root hairs on the 

shown on a larger scale in . , . , , 

the next figure. periphery of the root. 

FIG. 9. Diagram to show 

Structure and Function of the Stem 

72. The structure of the stem differs somewhat in its sig- 
nificant features from that of the root. In a cross section of 
a young stem we find, as in the root, a ground tissue of thin 




walled spherical or polygonal cells. Such tissue is generally 
termed parenchyma. In this case it occupies the axis of the 
stem and forms the pith. There are also radial extensions 
of the parenchyma from the pith toward the surface of the stem. 
The disposition of the parenchyma in a cross section might 






FIG. 10. Cross section of a typical dicotyledon stem from the pith to the 
epidermis and comprising one vascular bundle. See preceding figure. 

therefore be likened to the hub and spokes of a wheel. Ir 
the position corresponding to the felloes of the wheel there is 
also more or less parenchyma. 

73. The tire of the wheel is represented by a single layei 
of brick-shaped cells whose outer walls are thickened and forn 



a continuous layer of smooth, tough and impervious cuticula. 
This layer of cells is the epidermis. It is sufficiently elastic 
to allow considerable expansion with the growth of the stem, 
but it may finally be ruptured and scale off, leaving the under 
parts exposed. 

74. The spaces between the spokes of the parenchyma wheel 
are occupied by a system of fibers and vessels known as the 
vascular bundles. The fibers are usually of two distinct 
types, one, known as bast, is found nearer the surface of the 
stem, while the other is the chief element of the wood and lies 


FIG. n. Diagrams representing the structural elements of the vascular 
bundles. A, A fibre of wood, or bast; B, one end of a tracheid, showing spiral 
markings; C, part of a trachea, or true vessel, with pitted markings and the 
remnant of the dividing wall which originally separated two of the cells which 
helped form the vessel; D, part of a sieve tube with the perforated cross wall 

nearer the pith. The bast fibers consist of greatly elongated 
cells with extremely thick walls. The fibers of the wood are 
similar but the walls are not so thick. The vessels are of several 
kinds; first, tracheides, consisting of single elongated cells 
whose walls are unbroken, but variously thickened in limited 
areas, forming rings, spirals, annular pits, etc. ; second, the sieve 
vessels, formed by rows of elongated cells placed end to end 
with the dividing walls perforated by pores forming a sieve; 


> a o " % * 

3 &*> ti i ,5 * " , 

*- C > ^ "^o*" 1 3 

FIG. 12. A diagram to show the character of the tissues and their disposition 
in a young stem of the typical dicotyledon type. (From Stevens.) 



FIG. 13. Diagram similar to the preceding but representing a later stage and 
showing the tissues formed by the cambium. (From Stevens.) 


third, the true vessels, which originate from rows of cells whose 
dividing walls disappear, leaving a continuous passage from cell 
to cell. The walls of the true vessels are also thickened in 
spiral lines and otherwise as in the tracheides. The tracheides 
and vessels lie on the side of the vascular bundle next the pith, 
while the bast and sieve vessels lie next the surface of the stem. 

75. The bast and wood portions of the vascular bundles are 
separated by a zone of very thin-walled cells. This is the 
cambium, the region in which the new cells are formed and added 
to the tissues on either side, increasing the thickness of the bark 
on one side and adding to the wood on the other. The delicate 
cambium is readily torn and forms the line along which the bark 
separates from the wood. 

76. The woody portions of the vascular bundles are arranged 
side by side around the pithy axis, thus forming the cylinder 
alluded to above (paragraph 36). 

77. By experiment it may readily be determined that the 
fluids absorbed by the roots rise through the stem through the 
vessels and cell walls of the wood and not through the bark. 
The same fact is demonstrated by the effect of girdling a tree, 
which operation does not prevent the rise of the sap nor cause 
wilting of the leaves. 

78. Other functions of the wood and bark will be noted 

Structure and Function of the Leaves 

79. In order to fully understand the function of the leaf and 
the important processes that take place within its tissues it is 
necessary to study the finer details of its structure by means 
of the microscope. Thus under moderate magnification a leaf 
seen in cross section presents the following essential elements of 
structure : 

80. Both layers of epidermis consist of a single layer of brick- 



shaped colorless cells, whose outer walls are thickened and 
cutinized, whereby they become tough and impervious. This 
modification of the cell wall is usually more marked in the case 
of the upper epidermis. The epidermis usually the lower, 
sometimes the upper, frequently both is pierced by numerous 

FIG. 14. Cross section of a typical leaf. Five stomata are shown in the lower 


pores, the stomata, which open into a system of intercellular 
spaces filled with air. The outside atmosphere is thus given 
free access to all parts of the mesophyll through the stomata 
and this system of intercellular air spaces. 

8 1 . The more compact upper layer of the mesophyll consists 
of cells elongated perpendicularly 

to the epidermis and arranged in 
ranks, whence they have received 
the name "palisade cells. " The 
lower, spongy layer of the meso- 
phyll consists of cells less regular 
in form and arrangement and 
more completely surrounded by 
air spaces, but otherwise like the 
palisade cells. 

82. The important characteristic 
of the cells of the mesophyll is the 
presence of numerous minute green 

granules embedded in the protoplasm. The granules are special- 
ized parts of the protoplasm and are called chloroplasts. The 
substance which gives them color is called chlorophyll. It is, 

FIG. 15. Surface view of the 
epidermis of a leaf showing several 
stomata. The guard cells are 


this chlorophyll which gives green plants their characteristic 
color. It may be extracted from green tissues by alcohol, in 
which it is soluble, the alcohol then becoming green and the 
chloroplasts colorless. 

83. The function of the various parts of the leaf may be 
determined by suitably conducted, simple experiments. 

84. The usual position of the stomata, on the underside of the 
leaf, indicates that the stomata are not organs for the absorption 

FIG. 1 6. Stereogram of leaf structure. Part of a veinlet is shown on the right. 
Intercellular spaces are shaded. (From Stevens.) 

of water. Besides, most leaves, due to the presence of a waxy 
secretion on the surface of the epidermis, do not wet and, con- 
sequently, water would not pass through the minute openings. 
The function of the stomata must be to permit an interchange 
of vapors and gases between the intercellular air spaces and the 
atmosphere; for by experiment it can be shown that water vapor 


is given off from leaves, but only from the surface provided 
with stomata, consequently the stomata must be regarded as 
the openings through which the vapor escapes. The term 
transpiration is applied to this process by which water in the 
form of vapor escapes from the leaves. 

85. If the atmosphere surrounding a green plant growing in a 
closed chamber and exposed to the sunlight be tested from time 
to time for carbon dioxide and 
oxygen it will be found that the 
percentage of the former gas 
decreases, while that of the 
latter increases. Other tests 
will further show that the car- 
bon dioxide is absorbed and 
assimilated by the leaf, in which 
process an excess of oxygen over 
that required by the plant is set 
free in the leaf, and, if the leaf 
is immersed in water, the oxy- 
gen may be seen to collect on FlG> I7 _^ Diagram of stomain 

the surface of the leaf in bub- open and closed condition (heavy 

lines represent stoma open). B, C, 
Dies. ihese gases are not ab- and D, successive stages in the de- 

sorbed or eliminated through 

epidermal surfaces having no 

stomata, consequently we must conclude that the stomata 

give passage to carbon dioxide and oxygen, as well as to water 


86. The rate of transpiration of water vapor is controlled 
by an automatic opening and closing of the stomata. Excessive 
transpiration results in the wilting of the leaf, which means that 
the cells having lost some water are less turgid. The cells which 
guard the stoma on either side are so constructed that with 
increased turgidity they open the stoma, while with loss of 
turgidity the stoma is closed. Of course the rate of absorption 


of carbon dioxide and the accompanying elimination of oxygen 
is also dependent upon the opening and closing of the stomata. 


87. The leaves of a green plant growing under normal con- 
ditions always contain starch when the plant has been exposed 
to sunlight for a time. The starch disappears at night or when 
the plant is placed in the dark. It also disappears if the plant 
is kept in an atmosphere which contains no carbon dioxide. 
Etiolated leaves contain no starch under any circumstances. 
It appears from these facts that starch is formed in the leaf 

only in the presence of chlorophyll, carbon 
dioxide and sunlight. The chemical formula 
for starch is CeHioOs, a carbohydrate deriva- 
tive formed by the combination of CO2 and 
H 2 0, thus, 6CO 2 +5H20 = C 6 Hio0 5 +602, the 
surplus oxygen being given off by the plant. 
It will be noted that the number of molecules 
FIG. 1 8. Starch of oxygen given off equals the number of 

ce a n n t S r'c OV linfs CO of molecules of c 2 absorbed, which means that 
growth. the volumes of the absorbed and eliminated 

gases are equal. 

88. The power of forming starch from inorganic matter is a 
property peculiar to green plants, because of their chlorophyll, 
and gives them the distinction of being the source whence all 
organisms derive their food, since starch is the proximate or- 
ganic form of almost all food substances. The starch is formed 
within, or in contact with, the chloroplasts and appears first as 
minute granules which grow by the addition of layers to the 
outside in such a way that the surface of the fully formed grain 
is marked by peculiar concentric lines. 

89. We now see whence the plant derives its large amount 
of carbon. From the formula for starch it follows that 4/9 of 


its weight is carbon, which is approximately the proportion of 
carbon in the total plant tissue. The CO 2 of the atmosphere 
finds its way, with the other constituents of the air, through 
the stomata of the epidermis, into the intercellular spaces of 
the leaf. From here it passes through the cell walls of the 
mesophyll by osmose and is then by photosynthesis converted 
into starch, the free oxygen passing out of the cell, also by 
osmose, to the air of the intercellular spaces and thus out of 
the leaf. This process must not be regarded as assimilation, 
since the substances absorbed have not been converted into 
living protoplasm nor built up into the structural elements of 
the plant. The starch is simply food material which has been 
manufactured by the plant from a substance which is not food. 
For CO 2 cannot be directly assimilated by protoplasm. 

90. Starch is practically insoluble in water at ordinary 
temperature, yet it quickly disappears in an active cell when 
photosynthesis is not going on. There is an active principle 
called a ferment present in the protoplasm, which corrodes the 
starch grain, wearing away the surface until finally it goes to 
pieces and disappears. In place of the starch a form of sugar 
is found, dissolved in the cell sap. This is formed directly from 
the starch by the addition to the molecule of a molecule of water, 
thus starch (C 6 HioO 5 )+H 2 = sugar (C 6 Hi 2 O 6 ), a substance 
readily soluble in water. This soluble food substance may be 
directly assimilated by the protoplasm of the cell in which it was 
formed, or it may be transferred to other cells by osmose. 


91. The fact that oxygen is liberated from the plant during 
photosynthesis must not be interpreted to mean that the 
plant does not need oxygen. As has been noted elsewhere, 
oxygen is necessary to the germination of seeds and it is as 
necessary to the growing plant. During photosynthesis more 


oxygen is liberated than is needed by the plant and the excess 
escapes. But during the night, or when for any reason no 
oxygen is set free in the tissues by the synthesis of starch, the 
plant absorbs oxygen directly from the atmosphere. This is 
at all times true of plants which contain no chlorophyll. The 
process of absorbing oxygen of whatever source, by vegetable 
tissues, is called respiration and is identical with respiration 
in animals. 

Translocation of Food Substances 

92. The midribs and veins of the leaf are continuations of 
the vascular bundles of the stem. Besides giving support to 
the softer tissues they also bring the leaf into communication 
with the rest of the plant through the vascular system, per- 
mitting the passage of liquids and gases between the leaf and 
the stem. 

93. Since starch and, consequently, sugar, are formed only 
in cells containing chlorophyll, all other cells must be dependent 
for their food upon those which contain chlorophyll. Conse- 
quently, in the larger number of plants, the leaves must elabo- 
rate all the food for the stem and root. Starch is frequently 
found in parts containing no chlorophyll. In such cases it 
has been formed from sugar by the action of colorless corpuscles, 
called amyloplasts, which differ from chloroplasts only in the 
absence of chlorophyll. The course of the sugar through the 
stem is chiefly along sieve vessels and the surrounding paren- 
chyma. In passing from cell to cell it is frequently converted 
into starch and then reconverted into sugar preparatory to the 
next osmotic transfer. 

Other Food Substances 

94. Besides the carbohydrates, starch and sugar, there are 
several other kinds of food substances elaborated in the leaf. 



=- A 


FIG. 19. Section of a grain 
of wheat. A, Pericarps and 
seed coats; B, layer of cells in 
the endosperm containing 
aleurone grains; C, cells of the 
endosperm containing starch 

Prominent among these are the various vegetable oils, which are 
also compounds of carbon, hydrogen and oxygen. Globules 
of oil may be found in the tissues of the leaf and in other parts 
of the plant, but it is especially in the seeds of certain plants 
that large quantities of oil are stored 
up, to serve the same purposes that 
is served by starch in other cases. 

95. Aleurone is a substance which 
contains, besides carbon, hydrogen 
and oxygen, also a small per cent, 
of nitrogen. It is therefore called a 
nitrogenous substance and is very 
much like albumen. It is soluble in 
water and consequently disappears 
when immersed in a watery solution, 
but by mounting tissues of dry seeds 
containing it in a medium like glyc- 
erine, the aleurone may be seen under the microscope in the 
form of small granules. 

Differentiation of Tissues 

96. All reserve food materials are ultimately converted into 
protoplasm and from protoplasm the various structural 
elements of the tissues are formed. Thus the undifferentiated 
cell walls of parenchyma consist of cellulose, C 6 Hi O 5 , a sub- 
stance having the composition of starch. The cellulose is 
formed from layers of protoplasm by a process of chemical 

97. By further alteration in the chemical nature of the cellu- 
lose walls by which the proportion of carbon is increased, the 
walls assume special characteristics ; the surface wall of epider- 
mal cells becomes cutinized (cutin), the walls of cork cells be- 
come suberized (suberin), and the walls of wood and bast 
fibres become lignined (lignin). 

4 6 


98. All parts of the plant are covered, at least during the 
early stages of their development, by a superficial layer of 
cells forming the epidermis. On parts exposed to the air the 
outer walls of the epidermal cells are cutinized, which renders 
them impervious and tough. These properties render the 
epidermis well fitted to prevent desiccation of the underlying 
tissues and to protect them from mechanical injury. 

99. On the smaller rootlets, which are always surrounded 
by the moist soil and hence not subject to either desiccation 

* ^ *"**'" 

FIG. 20. The epidermis of various plants showing different degrees of cu- 
tinization (in black). A, Leaf of Avicennia, a Xerophyte; B, the epidermis of 
an apple (fruit); C, petal of Japan quince; D and E, upper and lower epidermis of 
leaf of Hibiscus Moscheutos; F, epidermis of leaf of prickly lettuce, Lactuca 
scariola, in the sun; G, same, in the shade. (From Stevens.) 

or mechanical injury, the epidermis is not cutinized, conse- 
quently it offers no obstacle to the transfusion of water and in 
fact is here specially modified for the function of absorption 
through the medium of the root hairs, which are only expansions 
of some of the epidermal cells. 

100. Structures called hairs are also developed on aerial 
parts of the plant. These assume as endless variety of forms 
and serve various functions. Some are glandular, others are 
organs for water absorption, and still others serve a variety of 



special functions. Most of those found on the leaf and stem, 
however, must be classed with protective structures, protecting 
the parts they cover from too intense sunlight, too rapid trans- 
piration, attacks of animals, wetting and frosts, etc. 

101. The epidermis is elastic and stretches to a remarkable 
degree as the parts covered by it expand with growth. But 
on the roots and stems of perennials 

the limit of elasticity is reached after 

a few years and then the epidermis 

gives way, breaking in various ways 

and exposing the tissues beneath. Its 

place as a protective structure is taken 

by the underlying layers of the bark 

which have then become modified into 

cork. This serves more efficiently the 

function of protection than did the 

epidermis, though at the expense of 

depriving the tissues beneath of the 

sunlight which had before been trans- 

mitted by the transparent epidermis. FlG . ._,, Ho oked hair 

The corky layers are thick and opaque, from the stem of Phaseolus 

i , . i multiflorus: 2, climbing hair 

though at the same time extremely on stem of Humulus Lupu- 
impervious, extremely poor conductors lus > 3, rod-like wax coating 

r ' r . on stem of Saccharum offic- 

Of heat, not readily yielding to the inarum; 4, climbing hair 

claw or tooth of beast or the beak of 
bird, almost valueless as food for 
animals and offering an excellent pro- 
tection against the attacks of fungous parasites. The outer 
layers of the cork are dead tissue, which usually splits into 
ridges as the stem, expands, and later the outer layers even 
scale off and drop away, while new layers are constantly 
forming beneath from the cork cambium. 

102. The most delicate tissues of the plant are those in which 
growth is taking place by the multiplication of cells. The three 

of Losa hispida; 5, stinging 
hair of Urtica ureus. (From 
Stevens, after deBary and 


chief regions in which this occurs are the centre of the bud, 
the cambium layer of the stem and roots and the root tip. The 

FIG. 22. Diagram to show the development of the tissues (differentiation' 
near the tip of a growing stem. The four figures on the right represent cross 
sections at different distances from the tip of the stem. The figure shoulc 
represent the tip of the stem covered by the young leaves. (From Stevens. ] 

growing tissue of the bud lies at the centre of a group of oldei 
structures and hence is not exposed except that the bud as a 



whole may suffer injury; and the cambium lies beneath the bark 
which gives it ample protection. But the tip of the root, as 
it grows, must push its way through the harsh soil and is there- 
fore provided with a special protective structure, the root cap. 
This is a conical mass of cells 
fitting over the tip of the root. 
As the rootlet pushes forward 
through the soil some of the cells 
of the root cap are rubbed off 
or destroyed, while others from 
beneath take their places, new 
ones being continually formed 
for this purpose at the point of 
growth in the base of the cap. 

Modified Roots 

103. In many plants, espe- 
cially among biennials and per- 
ennials, the roots show peculiar- 
ities of form and structure which 
cannot be accounted for with 
reference to the usual functions 
of roots, viz., those of absorption 
and anchorage. These modifi- 
cations are often in the nature 
of enlargements, as in the case 
of the turnip and sweet potato. 

year such roots give rise to new shoots from undeveloped or 
adventitious buds. The root shrivels as the shoot grows 
because of the gradual absorption of the contained food store, 
the enlargement being due to the accumulation of starch or 
other elaborated food substances. 

104. A less common type of root is the prop root, which 


FIG. 23. Longitudinal section of 
the tip of a rootlet with the root cap. 
The lower third of the figure is the 
cap. The region of growth (multi- 
plication of cells) is indicated by the 
small size of the cells. The black 
dots are the nuclei. 

In the spring of the second 


springs from the stem above ground, or even, in some cases, 
from the branches, and grows down to and into the soil. Such 
roots are found in special cases in which the plant would other- 
wise be top-heavy for its basal root system. The prop roots 
are primarily for anchorage though they may also serve for 

105. Another form of modified root is found in certain climb- 
ing plants which have roots springing from the aerial parts of 
the plants. These aerial roots serve as hold-fasts, penetrating 
the superficial layers of the bark of the tree, or crevices of the 
rock, to which the plant clings for support. 

1 06. The function of the aerial roots of epiphytes, so common 
in humid climates, is not only to attach the plant to its host 
but also to absorb moisture. In some cases the moisture is 
absorbed directly from the atmosphere; in others, it is drawn 
from the sponge of decaying leaves and other vegetable sub- 
stance which collects among the tangled mass of roots. In the 
latter case the absorbed water is likely to contain more or less 
nourishing matter, extracted from the humus. 

Modified Stems and Branches 

107. Stems are frequently so much reduced in length that 
the leaves seem to spring directly from the roots. In such 
"stemless plants," however, the conical or disc-shaped surface 
from which the leaves arise must be regarded as the stem, at the 
apex or centre of which the terminal bud will always be found. 
Many biennials remain "stemless" during the first season, but 
during the second period of growth produce a normal stem 
and branch system by development from the terminal bud. 

1 08. Another type of stem is that characteristic of the climb- 
ing and trailing plants. In these the stem is too slender to 
maintain itself in an erect position. The climbers depend on 
other objects for support, the stem serving merely as the 
conducting system connecting roots and leaves. In the 


case of trailers the plant is enabled to secure a large light 
exposure by spreading over a large surface of ground. The 
stem, in this case also, serving only the function of conduction. 
Climbers from their habit are adapted to forested regions, 
while trailers flourish in open ground. 

109. In many cases trailing stems take root at the nodes. 
Such stems are called runners, or stolons. The object of such 
a habit may be simply supplementary to the function of the 
basal root system or else, if the stem also produces a system 
of branches at the nodes, it may result in the production of 
new plants an asexual method of reproduction. In this case, 
after the young plant has become firmly established the stolon 
connecting it with the parent may die, leaving the young plant 
independent. If the connecting stolons persist it is possible 
that the associated individuals may be of mutual physiological 
assistance at critical times in the way of furnishing each other 
nourishment, etc. It is certainly true that such plants growing 
on a shifting soil are of great mutual assistance in holding each 
other in place and thereby also holding the soil. Stolons may 
be either above or under the ground. 

no. Underground stolons are sometimes greatly enlarged 
at the end, forming tubers. This is due to the accumulation of 
food substances for the purpose of storage, which is one of the 
normal functions of the stems of perennial plants. In the larger 
perennials the normal stem is large enough to provide sufficient 
storage and consequently no special enlargement is necessary. 
In the case of the smaller herbaceous perennials, however, there 
is insufficient storage room provided by the comparatively 
small normal stem and therefore the storage stems are enlarged. 
Moreover, since the normal herbaceous stems usually do not 
survive the winter, the stems which are modified for storage are 
developed underground, where they are protected, and whence 
they put forth shoots in the following spring from terminal 
and axillary buds. 


in. Other types of underground storage stems are common. 
The root-stock differs from the tuber in that it has no slender 
connecting stem but is thickened throughout its length. It 

FIG. 24. Young shoots of the common cactus, Opuntia, showing the small, 
conical leaves. These soon disappear. Note that the spines develop in the 
axils of the leaves. X2/3. 

usually persists from year to year; a new segment consisting 
of one or more nodes, being added each season. When the 


root-stock is much shortened and vertical in position in other 
words, merely an extremely short stem it becomes a corm. 

112. Many plants inhabiting semi-arid regions are adapted 
to the recurring long periods of drought following the brief 
periods of rainfall by the habit which they have assumed of 
storing up water. The stems form the reservoirs and are con- 
sequently of much greater bulk than the other functions of the 
stem would demand. 

113. A less common type of modified stem is one in which 
the branch takes up the functions of the leaf. In this case the 
branch may become flattented and like the leaf in other respects. 
Thorns in certain cases are also modified branches. 

Modified Leaves 

114. Besides the endless diversity of form assumed by foliage 
leaves, there are also a number of leaf types in which the function 
of photosynthesis has been entirely lost. Such, for example, 
are the bud scale-leaves, which serve as protective organs, and 
the scale-leaves of underground stems, which are functionless 
rudiments. Certain kinds of thorns and tendrils, are modified 
leaves or parts of leaves. Even the function of food storage 
is sometimes assumed by leaves. The blade of the leaf in one 
group of plants is entirely wanting and its function is performed 
by the petiole, which is flattened laterally and has the appear- 
ance of a leaf blade turned into the vertical plane. 

Homology of the Flower 

115. While the modification of the type forms mentioned 
in the preceding paragraphs are all quiet common, still they are 
in every case limited to a small minority of plant species. There 
is, however, a most important and interesting kind of modi- 
fication which is practically universal among seed-bearing plants. 


This is the modification of a branch, involving both stem and 
leaves, which results in the structure we call the flower. 

1 1 6. A leaf bud and a flower bud are in all essential points 
alike. There is a very short central axis around which are 
arranged the rudimentary leaves in regular whorled or spiral 
order. In the development of the leaf bud the axis elongates, 
separating the leaves, while the latter expand and assume the 
form and color of the typical leaf. In the case of the flower 
bud, however, the axis does not elongate regularly throughout 
its length. It may remain very short, in which case the flower 
remains sessile. If the axis elongates at all the elongation 
affects only a limited part, by which a stem (pedicel or peduncle) 
is formed. At the top of this stem the flower leaves still remain 
in closely set whorls or circles. 


117. The homology of flowers is also shown by their position 
on the stem and their groupings. When flowers occur singly 
they are either terminal or axillary and hence arise from ter- 
minal or axillary buds, or else they spring from accessory buds. 
In either case their origin is the same as that of branches. 
Whenever a flower terminates an axis the growth of that axis 
ceases with the growth of the flower, consequently further 
growth of the plant must proceed from another bud. 

118. Flowers which occur in groups may be divided into two 
classes, depending upon whether the first flower to appear is 
terminal or lateral. In the former the grouping of the flowers 
is called a determinate or cymose inflorescence. In this case 
the first terminal flower is followed by two opposite, lateral 
ones which grow beyond the first, leaving it apparently in the 
angle of two equal lateral branches. This is a simple cyme. 
If the two lateral flower stalks also each put out, in a similar 
way, a pair of lateral flowers, the cyme becomes compound. 



119. When the first flower of an inflorescence is lateral the 
terminal bud continues to grow for some time and new flowers 


\/ Nl/ 


FIG. 25. Cymose inflorescences. F, A terminal flower; G, a simple cyme; H, a 

compound cyme. 


? c 


3 / 





' c 

/" c 









FIG. 26. Types of racemose inflorescence. A, A raceme; B, a spike; C, a 
catkin; D, a corymb; , an umbel. The flowers are represented by circles; 
the age of the flower is indicated by the size. 

continue to develop above the first one along the main axis. 
Such an inflorescence is indeterminate and is called a raceme. 


If the flowers of the raceme are sessile the inflorescence is a spike; 
if they are stalked it is a true raceme. A scaly, pendulous, 
deciduous spike is a catkin. If the older flowers of a raceme 
rise to the level of the terminal one because of their longer stalks, 
and thus form a flat topped cluster, we have a corymb. When 
the rachis is much shortened and the flowers equally stalked 
the inflorescence is an umbel and a similar condition of the 
rachis with sessile flowers is a capitulum. 

120. That an inflorescence is made up of a system of branches 
is further shown by the fact that each flower springs from the 
axil of a bract, or rudimentary leaf. These are often green, 
but sometimes scale-like or chaffy. There is frequently a 
series of such bracts at the base of an inflorescence forming an 
involucre. This is especially true of capitulate inflorescences. 

Structure of the Flower 

121. A complete flower has four sets of floral leaves, all more 
or less completely transformed for special functions and bearing 
little resemblance to the foliage leaf. The extreme diversity 
of species with respect to the characteristics of the flower makes 
it impossible to give any general description of it which will 
apply to all cases. However, an ideal flower with which all 
others may conveniently be compared may be described as 

122. That part of the flower stem which bears the leaves is 
so much shortened that it forms practically a flat surface, the 
receptacle, upon which are borne the concentric circles of floral 
leaves. The outermost or lowest, of these circles is called the 
calyx. It is formed of from three to five leaves, which are green 
and enclose the other parts in the bud. The next circle within 
this is composed of a similar number of parts, characterized by 
some color other than green. This circle is called the corolla, 
and corolla and calyx together are sometimes called the peri- 



anth. Neither calyx nor corolla are essential parts of the flower. 
One or both may be wanting without thereby impairing the 
function of the flower. 

1 23 . The third circle constitutes the andrcecium and is made 
up of parts called stamens, which ordinarily have little resem- 
blance to leaves. There is usually a slender stalk (the filament) 

FIG. 27. Diagrams of floral structures. A shows the relations of the floral 
parts in a hypogynous flower; B, the same in a perigynous flower; C, the same in 
an epigynous flower; D, a stamen; E, a simple pistil in longitudinal section; F, 
the same in cross section; G, transitional forms between true petals (left) and 
true stamens (right); H, slight union of two carpels to form a compound pistil; 
7 and /, union of carpels more complete; K and L, cross sections of compound 
pistils, of three carpels. In B: a, stamen; b, petal; c, sepal; d, pistil; e, receptacle; 
/, pedicel. In D: a, anther cell; b, connective; c, filament. In E: a, stigma; 
b, style; c, ovules; d, ovary. 

at the summit of which is attached a double sack-like organ 
(the anther) containing a powdery or granular substance (the 
pollen) . 

124. In certain flowers the stamens bear a close resemblance 
to a leaf. In such cases the filament is leaf-like in form and the 
cells of the anther are borne on its edge. The number of sta- 
mens is frequently the same as, or a multiple of, the number 
of parts of the calyx or of the corolla, but it may vary from one 
to many. 


125. The gyncecium is the organ or set of organs formed by 
the fourth or inner circle of floral leaves. The individual 
leaves (carpels) composing it may be more or less united to 
form a single structure or, not infrequently, the number of 
carpels may be reduced to one, which then occupies the centre 
of the flower. In case there is only one carpel, or if the carpels 
are separate, each one constitutes a simple 
pistil, which must be regarded as having 
been formed by the rolling of the blade of 
the carpellate leaf, so that its opposite 
edges meet and unite and thus enclose 
a flask-shaped cavity. The pistil thus 
formed may be described as consisting 
of the ovary the cavity of the flask with 
its enclosing walls, the style the neck of 
the flask and the. stigma, a slight glandu- 
lar enlargement at the top of the style. 
(See Fig. 27.) 

126. Within the cavity of the ovary 
and attached to its walls are one or more 
minute bodies, the ovules, which are 
destined to develop into the seed. The 
specialized part of the ovary wall to which 
the ovules are attached is the placenta. 
(See Fig. 27.) 

127. Pistils are frequently compound, i. e., made up of more 
than one carpel. In such cases there may be various degrees 
of fusion of the component leaves, ranging on the one hand from 
a slight external union of the ovary walls to such complete 
fusion on the other, that the only evidence of its compound 
nature is to be found in the number of placentae. The number 
of pistils in one flower varies from one to many. 

128. Both andrcecium and gyncecium are essential and with- 
out either one the flower is incapable of performing its function. 

FIG. 28. One of the 
four leaf-like carpels of 
the Chinese parasol tree 
(Sterculia) with several 
seeds attached to its 
margins. The carpels 
separate early and as- 
sume a leaf-like form. 



There are many plants, however, in which stamens and pistils 
are not found in the same flower. In such cases there are 
two kinds of flowers, one staminate, the 
other pistillate, both found on the same 
plant (monoecious) or separate plants 
of the same species (dioecious). 

129. The number of deviations from 
the ideal flower just described are too 
many to be enumerated, but it will be 
necessary to indicate the most impor- 
tant ones in order that homologies may 
be recognized. 

130. The receptacle may be either 
convex, flat, or concave. In the latter 
case the edges of the receptacle may 
rise so high around the gyncecium as 

to entirely enclose it within the con- 

J m 

cavity. (See Fig. 27.) 

131. The calyx is very rarely want- 
ing since its function is that of protec- 

sepals, but frequently there is a more 

FIG. 29. Diagram of a 
pistil with one ovule in the 

attached by a stalk, the 

funiculus, and is provided 

Its parts may all be separate with two protective layers, 

or less complete union of the edges of the seed coats. The em- 

,, i -.LI i .LI r bryo is developed from the 

the sepals with each other so as to form egg nuc i e us (small circle) 

a cup or tube (calyx gamosepalous) . whi( * lies in the embryo 

/ sac (large oval). The em- 

132. The corolla is often entirely bryo sac is embedded in a 

wanting. In other cases it is present, ^ *&& 

but inconspicuous. Usually, however, bryo is nourished by the 

. . 11 . . contents of the embryo sac 

when the corolla is present it is very and the nucellus. Reserve 
because of the size and 


color of its parts. Like the Calyx it sperm while that found in 
i ! - c i. .. the nucellus is perisperm. 

may be made up of distinct parts, or 

the parts may be more or less united into a single structure 

(corolla gamopetalous) . 


133. The stamens may also be distinct or united into one 
(monodelphous) , two (diadelphous) or more groups, the union 
being due to the cohesion of either filaments or anthers. 

134. A union of floral organs occurs, not only between mem- 
bers of the same series, but also between adjacent series. Thus, 
the petals may be' united with the calyx cup in such a way that 
they seem to spring from the edge of the cup instead of from 
the receptacle. The stamens, likewise, may be adnate to the 
petals or fused with the calyx tube and thus, like the petals, 
apparently inserted upon it (perigynous) . Still greater fusion 
may occur and calyx, corolla, and stamens all be more or less 
united with the ovary and thus apparently inserted on its side 
(perigynous) or top (epigynous) instead of on the receptacle. 
In the latter case the ovary is said to be inferior. 

Function of the Flower 

135. The function of the flower is to produce the seed, but 
this is accomplished only by the conjoint action of pollen and 
ovule. Under normal conditions, the ovule at a certain time 
begins a series of developmental changes by which it finally 
becomes a seed. This latter phase of its development is begun, 
however, only after pollination and fertilization. Pollination 
is the transfer of pollen from the anther to the stigma by the 
wind, by insects, or through some other agency. After reaching 
the stigma the pollen grain develops a tubular outgrowth which 
penetrates the tissues of the stigma and style growing down 
to and into the ovule. A certain nucleus of the pollen tube 
then fuses with a similar nucleus of the ovule. This fusion of 
elements from the pollen grain and ovule is known as fertili- 
zation, because, as a result of it, the ovule is stimulated to 
further development which finally results in the seed, whereas, 
if the fusion does not occur, there is no further development of 
the ovule and no seed is produced. 



136. A necessary preliminary to fertilization, however, is 
pollination, which is brought about in many different and often 
remarkable ways, all of which illustrate most clearly the nice 
adaptations, or correlations, of plants with other organisms and, 
in general, with their environment. 

137. It may first be noted that most flowers are so organized 
as to effectually protect their pollen from wetting. This means 
in many cases merely that the flowers do not open except in 
fair weather and then require only a few minutes or hours 
for the accomplishment of pollination. Other flowers close 
at night or during threatening weather, i. e., the petals assume 
a position such as to protect the stamens from rain or dew. 
In other cases the petals, some or all, are so disposed as to give 
shelter to the stamens; and frequently the flowers are pendant, 
so that the stamens are sheltered even when the petals are widely 

138. It is a well recognized biological principle that cross- 
fertilization, i. e., fertilization by pollen from another plant of 
the same species, results in more vigorous offspring than 
does self-fertilization fertilization resulting from the union of 
elements of the same plant. Accordingly we find that plants 
are so organized as to favor cross-fertilization. 

139. One of the most important agencies of pollination is 
the wind. The plants for which the wind performs this service 
all have small and inconspicuous flowers, i. e., the petals are 
either wanting or, if present, are small and not brilliantly 
colored. The pollen in such plants is light and powdery and is, 
therefore, easily carried by the wind, sometimes to long dis- 
tances. It is produced in great quantities and, as it is wafted 
along on the wind in clouds, some grains are likely to fall upon 
other flowers of the same species and be held there by the 
adhesive stigma. 



140. To the fact that anemophilous flowers are inconspicuous 
must be added the evidently related facts that such flowers are 

FIG. 30. The inflorescence of Polygala. The flowers are clustered in the axils 
of the whorls of leaf-like bracts. These bracts are violet colored and render the 
inflorescence very conspicuous. 

also devoid of odor and secrete no honey. These facts become 
significant when we learn further that all flowers which are 
conspicuous because of the color of the corolla or other parts, 


or are scented, or secrete honey, have the office of pollination 
performed for them by insects which visit them for the pollen 
or honey; the pollen as well as the honey being used by insects 
as food. The colors and scents of flowers are evidently related 
to the senses of sight and smell of insects and serve to attract 
the insects to them. 

141. Insects on visiting the flowers necessarily come in 
contact with the anthers and some of the pollen clings to the 

FIG. 31. The dichogamous flowers of Polygala. The flower A shows the 
anthers protruding from the hood. In B the stigma has advanced and is ready 
for pollination. 

insect's body; for the character of the pollen grains in such 
plants differs from the powdery pollen of anemophilous plants 
in that it is sticky by virtue of a viscid or oily coating, or 
because of the prickles, grooves, ridges or other structural 
peculiarities of the wall of the pollen grain which cause it to 
ciing more readily to the hairs of the insect's body. Now as 
me insect moves on to another flower it will in all probability 

6 4 


FIG. 32. Bumblebees and wasps which carry pollen for Polygala. The hairs on 
the thorax of all these insects were covered with pollen. 


brush off some of this pollen upon the parts of the flower with 
which it comes in contact. Because of its position and because 
of the character of the stigmatic surface, the stigma will be 
most likely to receive and retain some of this pollen, and thus 
pollination will be accomplished. 

142. The question now arises, is not pollination by wind or 
by insects as likely to result in self-fertilization as in cross- 
fertilization? The study of further facts will lead us to answer 
this question emphatically in the negative. The facts are these : 
In the case of anemophilous plants the flowers are either dioe- 
cious, monoecious, or dichogamous. In the first case, of course, 
self-fertilization cannot occur. In the case of monoecious 
plants the staminate and pistillate flowers are not on the same 
level, consequently the pollen floating horizontally on the wind 
is not likely to fall upon any of the pistillate flowers of the same 
twig. If the flowers are hermaphrodite they are also usually 
dichogamous, which means that either the andrcecium or the 
gyncecium matures first and hence the pollen cannot fertilize 
an ovule of the same flower. 

143. Entomophilous hermaphrodite flowers are usually 
either dichogamous, dimorphic or else by movements of stamen 
and pistil a result is brought about which is practically equiva- 
lent to that attained by dichogamy. In the case of dimorphic 
flowers there are two kinds of flowers which differ with respect 
to the length of the style and stamens, and the position of the 
stigma in one form of flower corresponds to the position of the 
anther in the other. The result of this is that when an insect 
visits the flower it receives pollen on that part of its body with 
which the stigma comes in contact when the insect visits a 
flower of the other type. 

144. Self-fertilization is also known to occur in many herma- 
phrodite flowers. However, it takes place, usually, only after 
the methods for securing cross-fertilization have been employed 
by the flower; the result being to insure fertilization in case 


cross-fertilization fails. Autogamy is secured in a great variety 
of ways. Some of these are; by movements of the anthers, 
by movements of the stamens or style, or both, by changes 
in length of stamens or style, by changes in the corolla, etc. 

FIG. 33. The inflorescence of the dog- wood. The flowers are small and 
greenish and occur in clusters. Beneath each group of flowers are four large 
white bracts which take the place of the petals in making the inflorescence 

145. Some plants develop seed from flowers which never open. 
Such plants also produce blossoms under favorable conditions 
and the cleistogamic flowers are to be regarded merely as a 
special form of autogamy. The corolla is reduced and 
other parts of the cleistogamic flower may differ from the 
corresponding parts of the blossoming flower. < 


146. It undoubtedly often occurs that various kinds of pollen 
fall on the same stigma. This is quite likely to be the case 
with anemophilous pollen, but occurs less frequently in ento- 
mophilous flowers because many species of insects confine their 
attention to one or a few species of flowers; and also many 
species of flowers are visited by only one or a few species of 

1 4 7. "When pollen from a distantly related plant falls on the 
stigma of a flower no fertilization occurs. If the pollen comes 
from a nearly allied plant, however, fertilization may take 
place and the resulting offspring will be a hybrid. But normally 
only pollen coming from a flower of the same species is effica- 
cious in producing fertilization. If several kinds of pollen fall 
on the same stigma at about the same time there may, therefore, 
be a selection of the kind proper to the plant. As between 
pollen from the same flower and pollen from another flower of 
the same species it is quite probable that there may, also, be 
a selection in favor of that yielding cross-fertilization. This is 
known to be the case in certain plants and, by analogy, it may 
occur in others. 

The Seed 

148. Fertilization accomplished, the development of the seed 
begins. The embryo itself is developed from the germ nucleus 
which results from the fusion of the fertilizing pollen nucleus 
and the egg nucleus of the ovule. But the germ nucleus is 
only a small part of the ovule. The other parts also grow as 
the embryo develops, and form the masses of reserve food and 
the seed coats. 

The Fruit 

149. While the ovules in the ovary are developing into seeds, 
changes are also taking place in adjacent parts of the flower 
changes which would not occur if the ovules failed of 


fertilization. Sometimes all the tissues of a flower cluster are 
involved, more frequently the receptacle or calyx, but always 
the ovary. The structures become enlarged and fleshy or 
indurated and modified in various other ways. The resulting 
structure or organ is called a fruit and its function always has 
relation to the function of the seed. 

150. With the ripening of the seed the plant seems to have 
fulfilled the object of its existence and soon dies, or if it is not 
an annual it becomes dormant until the following season, when 
another period of growth is closed by the ripening of the fruit. 
Occasionally the fruit is not matured until the following season, 
in which case the activity of the plant during the first season 
is devoted to the storing up of reserve food, which is then 
used in the development of the fruit during the second 

151. The distinction between seed and fruit, then, lies in 
this, that the seed is only that part which develops from the 
ovule, while the fruit includes the seed and consists besides of 
the modified ovary and frequently other adjacent parts of the 
flower, which finally together constitute the seed-containing 
organ of the plant. 

152. Simple fruits are either fleshy or dry, and the latter 
are either indehiscent or dehiscent, hence the following classes 
of fruits are recognized: 

153. A berry is a fleshy fruit composed wholly of the peri- 
carp, or of the pericarp and the adherent calyx-tube. 

154. The drupe, or stone fruit, is also a fleshy pericarp, the 
inner layer of which is stony. 

155. The pome is a fleshy fruit derived from the concave 
receptacle which encloses the dry papery pericarp. 

156. An achene is a dry indehiscent fruit derived from a 
simple pistil and containing only a single seed. 

157. A caryopsis resembles an achene, but has the seed coats 
intimately united with the walls of the ovary. 


158. The nut also resembles an achene, but is derived from 
a pericarp consisting of more than one carpel. 

159. A samara is an indehiscent fruit with winged appendages. 

1 60. A schizocarp is a compound fruit which splits when 
ripe into two or more parts, each resembling an achene. 

161. A follicle is a dry fruit derived from a simple pistil and 
opens when ripe by splitting down one side. 

162. A legume, or pod, is like the follicle, except that it splits 
down both sides of the carpel. 

163. A capsule is a dry, dehiscent fruit derived from a com- 
pound pistil and opens by splitting down the side, by separating 
a lid from the top, by opening of small pores or otherwise. 

164. Aggregate fruits are those which are made up of the 
numerous distinct carpels of a single flower adhering together 
to form a single mass, or sometimes held together by the 

165. Multiple fruits are composed of the combined carpels 
and coherent parts of a number of flowers held together by 
the common receptacle. The common receptacle may be 
either convex or concave, in the latter case enclosing the 

Seed Distribution 

1 66. All the elaborate adaptations of the plant contribute 
directly to the one end that seed may be produced from which 
a new generation of the species may proceed. However, to 
produce mature seed is not of itself a guarantee that from that 
seed a new plant will spring. The seed must be brought to a 
spot where the conditions are favorable for its germination and 
development. But because of constantly changing conditions 
this is frequently not the case at the place where the seed was 
brought to maturity. The seeds must be scattered abroad in 
order that some may by chance fall upon good ground. 

167. Some seeds are so small and light that they are readily 


carried by the wind to great distances. Larger seeds in many 
cases have special contrivances in the form of sails, parachutes, 
or feathery or hair-like appendages which offer such a large 
surface to the wind that they may also be carried by it in spite 
of their larger size. Seeds swallowed by animals are frequently 
not digested and may be carried abroad by this agency. 

1 68. But aside from the protection which the fruit tissues may 
give the seed in some instances, it is often the function of the 

FIG. 34. A leaf of Bryophyllum developing new plantlets by budding at the 

edge of the leaf. 

fruit to provide for the dissemination of the seed. This object 
may be accomplished in an endless variety of ways; sometimes 
by mechanically scattering the seed when the fruit opens; 
sometimes by the development of hold-fast organs which cause 
the fruit to cling to passing animals; or again by means of para- 
chutes and sails by which the fruit is carried on the wind; or by 
floats on which the fruit drifts with the current of water. Edi- 
ble fruits of all sorts are carried by animals from place to place 
and the seeds scattered in this way. In some cases the larger 
part of the plant is concerned in the process of scattering seed. 


169. It is no t in all cases necessary that a seed should be formed 
in order that a new plant may be developed. Many peren- 
nials also multiply by a process called budding, which consists 
essentially of the development from some part of the parent 
stock of a shoot which ultimately becomes an independent 
plant. The shoot may spring from the roots, from underground 
stems, from runners or branches where they touch the sub- 
stratum or even from leaves. Some species belonging to the 
group of seed-bearing plants have adopted this method of repro- 
duction almost to the exclusion of the formation of seeds. 

Classes of Plants 

170. All the so-called flowering plants have one character- 
istic in common, which is the formation of a reproductive body, 
the seed, developed from the ovule after fertilization, and con- 
sisting essentially of an embryo enclosed in a protective seed 
coat. This group of plants is called Spermatophytes and con- 
sists of two divisions, the Angiosperms and the Gymnosperms. 

171. The Angiosperms are those seed-bearing plants in which 
the ovules are enclosed in the cavity of an ovary. Of these 
there are two classes, the Monocotyledons and the Dicotyle- 
dons. The difference between these two classes is shown 
in the following table: 



1. Two seed leaves. i. One seed leaf . 

2. Leaves netted veined and 2. Leaves parallel veined with 
with broken margin. ^margin entire. 

3. Parts of flowers in 45 or 55. 3. Parts of flowers in 35. 

4. Vascular bundles of the 4. Vascular bundles of the stem 
stem in a single circle form- scattered and no distinction of 
ing two concentric cylin- wood and bark. 

ders of wood and bark. 
Either of the characters, 2, 3, or 4, may in some cases fail to apply. 


172. The grasses, sedges, lilies, palms and orchids are the 
most important groups of the Monocotyledons. The Dicoty- 
ledons include most of the remaining seed-bearing plants except 
the "evergreens." 

FIG. 35. Inflorescences of the pine, i, Terminal twig; 2, ovulate cone; 3, 
staminate cone; 4, two-year-old cone. 


173. The Gymnosperms are distinguished from the Angio- 
sperms by the fact that the ovules are not enclosed by the walls 
of an ovary, but are simply covered by a scale. To this group 
belong the cone-bearing "evergreens;" as e. g., the pine, cedar, 
yew, larch, and spruce. 


174. Not all plants produce seed. There is a great variety 
of organisms which are not included in the groups so far consid- 


ered; e. g., ferns, mosses, sea- weeds, toadstools, molds, etc. 
These are all grouped together under the name of Cryptogams, 
but it is not thereby meant to indicate that there is a close 
relationship between the various members of the group. It 
signifies only that the members of which it is composed do not 
bear seed. As a whole the Spermatophytes are much more 
complex and for certain reasons are regarded as of a higher order 
than the Cryptogams. 

175. Of the Cryptogams, the highest class those most 
nearly resembling the Spermatophytes are the Pteridophytes, 
including the ferns and their allies. Most of these have an 
underground stem (rootstock or rhizome) with a system of true 
roots and a series of leaves held aloft on long petioles or stipes. 
The microscopic structure of the organs, too, resembles in a 
general way that of similar organs in the higher plants. The 
common ferns, the scouring-rushes and the club-mosses are 
familiar examples of this group. 

176. Next in order below the Pteridophytes come the Bryo- 
phytes, to which group belong the mosses and liverworts. 
These plants are all small. The moss plant consists of a slender 
stem, with scale-like leaves, but no true roots, and there are no 
well-developed vascular bundles in the stem. In liverworts 
there is usually no distinction of stem and leaf. The body of 
the plant consists simply of a flat expanse of green tissue. In 
place of roots the mosses and liverworts have organs which 
resemble root hairs, and are called rhizoids. 

177. All plants not included in the foregoing groups are 
classed together as Thallophytes a large and heterogeneous 
group which comprises all the lower or simpler plants. The 
body of a Thallophyte is never differentiated into root, stem 
and leaves, as is usually the case in the higher groups, and there 
are more exact distinctions to be observed in the methods of 
reproduction. The Thallophytes are divisible into two very 
distinct groups, algae and fungi, which are distinguished by the 


presence of chlorophyll in the former group and its total absence 
in the latter. Most of them are small, many are microscopic in 
size, but there are a few marine algae which are extremely large. 

178. The algae are found either in the water or else in moist 
places, for they have no elaborate protective structures which 
would prevent desiccation. There are many kinds which con- 
sist of only a single cell, others of similar cells arranged in rows 
or filaments. In others, again, the cells are arranged in sheets 
or masses having more or less definite forms. One group of 
marine algae in which the structure is rather complex is charac- 
terized by a reddish color, due to the presence of a red pigment 
in the protoplasm, which to some extent obscures the green of 
the chlorophyll. Another group of marine algae, simpler in 
structure, is similarly characterized by a yellow pigment which 
gives the plant a brownish color. A small group of extremely 
simple filamentous or unicellular algae is characterized by a 
blue-green color due to the presence of a blue pigment. There 
are many algae, however, which are neither red, brown nor 
blue-green, but have the yellowish-green color characteristic of 
chlorophyll. These vary in complexity of structure from the 
simplest to the most complex. The blue-green and the green 
algae comprise both marine and fresh water forms. 

179. The fungi vary as greatly in regard to complexity of 
structure as do the algae and may be regarded as a parallel 
series, differing chiefly from the algae in those points which are 
dependent on the presence of chlorophyll. Since they are 
destitute of chlorophyll, the fungi (excepting perhaps some of 
the lowest forms) cannot assimilate carbon dioxide and con- 
sequently are either saprophytic, i. e., nourished upon waste 
organic matter, or parasite, i. e., nourished upon the tissues of 
other living organisms. Some of the most familiar of the 
higher fungi are the toadstools, mushrooms, shelf-fungi, puff- 
balls, smuts and rusts of grasses, " cedar- apple," ergot, black- 
knot of plum trees, mildews, molds, yeast, etc. 


1 80. The lowest fungi are the bacteria, a large and impor- 
tant group, though made up of the simplest and minutest of 
all organisms. The bacteria are minute unicellular or fila- 
mentous organisms, so simple in structure that the cell con- 
stituting an individual seems to be devoid of even the nucleus. 
To this group belong the germs of many diseases, and the active 
agents in various processes, such as putrefaction and decay, 
souring of milk, acid and vinous fermentations, etc. 

181. Cryptogams reproduce by means of spores instead of 
by seeds. Spores are single cells specially set apart by the plant 
for the purpose of reproduction. In some cases they are formed 
by the union of two elements, as in the process of fertilization 
in Spermatophytes. Another kind of spore is formed merely 
by the separation from some part of the parent plant of a 
single cell, which has the power of developing a new plant with- 
out fertilization. Some of the lowest, simplest Cryptogams, 
consisting of a single cell, multiply merely by the division of the 
cell into equal halves (fission). 


182. In our study of the development, form, structure and 
life processes of a plant we have confined our attention almost 
entirely to the kinds of plants with which we have been most 
familiar, i. e., such as grow in soils that are at least moderately 
productive to the agriculturist and in climates which are the 
most habitable to man, neither extremely cold nor hot, nor ex- 
tremely wet or dry. And besides, we have limited our study 
to the independent, chlorophyll-bearing plants. In these we 
have seen with regard to every feature of the plant's organiza- 
tion a remarkable adjustment to its external conditions of 
existence, or, in other words, adaptation to environment. This 
has been so apparent at every turn that one might well regard 
it as a law of nature. However, if there be such a law, it must 

7 6 


apply to all plants in all circumstances under which they are 
found to thrive, although the environment may be very different 
from that which is normal to the plants we have been consider- 
ing. In order, then, to test the validity of this law, let us ex- 
amine the flora of localities which present conditions different 
from those we have already considered, 

FIG. 36. A tree deformed by the action of the wind and salt spray. The 
buds are continually killed on the windward side. Coast of North Carolina. 
A similar effect is produced by the combined action of wind and cold, as on high 
mountain summits. 


183. With regard to conditions of moisture, plants have been 
grouped into three classes, mesophytes, hydrophytes and 
xerophytes. Mesophytes are the plants which grow normally 
under conditions of moderate supply of moisture, and hence 
include all those which we have heretofore been studying. 
Hydrophytes are plants which grow in the water, or, at least, 
in very wet soils. Xerophytes are the plants peculiar to arid 

184. Among hydrophytes we may have, first, those plants 


which grow entirely submersed either in the sea, in fresh water 
streams or in quiet ponds. The most striking peculiarities 
common to plants living under such conditions is the almost 
complete absence of mechanical supporting tissue. Almost 
without exception, submersed aquatics are not rigid enough 
to support their own weight when taken from the water. 
Obviously the buoyancy of the water makes such a highly 
developed supporting system superfluous. 

185. Some aquatics utilize the buoyancy of the water for 
support by specialized bladder-like floats which represent 
modified leaf blades, petioles, or other organs. Very generally, 
also, the tissues of such plants contain extensive systems of 
passages filled with air. These serve not only to aerate the 
tissues, but at the same time act as floats. 

1 86. Other characters common to plants of this class are the 
undeveloped condition of the root system, which usually serves 
only as a hold-fast, and the absence of root hairs. The absorp- 
tion of water is carried on chiefly by the epidermis of the stem 
and leaves. For the epidermis, not being exposed to the dry 
air, is not cutinized and, consequently, is in condition to serve 
the function of water absorption. 

187. The leaves of submersed aquatics are commonly very 
narrow or finely divided. This offers several advantages under 
the conditions; the ratio of absorbing surface is increased, 
mutual shading lessened and there is less resistance to currents 
of water which would tend to dismember the plant. Besides, 
in a submersed plant, there would be no apparent advantage 
offered by a broad leaf over an equal expanse of narrow leaves. 

1 88. Plant surfaces continually in contact with water have 
no stomata, hence the gases absorbed in the case of submersed 
aquatics are taken from the water by osmose. 

189. In marked contrast with the finely divided leaves of 
submersed plants are the broad leaves of the floating aquatics. 
The under surface of leaves of this type is destitute of stomata, 


but the upper, exposed surface, has the stomata and an epider- 
mis like that of the mesophytes. These plants grow only in 
quiet, shallow water. They^are firmly rooted in the mud, 

FIG. 37. Salicornia ambigua, a xerophytic plant found in salt marshes and 
sandy beaches of the Atlantic sea-board. The leaves are rudimentary. Xi/2. 

from which the long stout petioles rise at an angle to the surface, 
where the broad leaf blades spread out in a single plane. These 
conditions allow the leaf to rise and fall with every change in 


the level of the surface. There can be no question of mutual 
shading and none of the considerations which gave advantage 
to the form of the submersed leaf can here apply. 

FIG. 38. A xerophytic habit of the prickly lettuce, Lactuca scariola. View 
as seen from the east. 

190. A few plants are capable of growing either entirely 
under water or with at least some of the leaves entirely above 


water. In these the effect of the water on the form of the leaf is 
clearly shown. Both kinds of leaf, the narrow ones below the sur- 
face and the broad ones above, may be found on the same plant. 

FIG. 39. Lactuca as seen from the south. This and the preceding figure 
show the leaves twisted into the vertical plane and bent toward the plane of 
the meridian. 

191. Along the border of quiet waters another type of hydro- 
phyte is to be found. The plants of this type stand in shallow 


water, firmly rooted in the mud, but are erect, self-supporting 
and rise tall and slender, high above the surface of the water. 
The special adaptation here is in the height of the plant, which 
permits considerable change in level of the water surface with- 
out drowning the plant. 

192. Marsh and swamp plants do not differ much from the 
mesophytes in structure, but nevertheless the continually 
saturated soil and other conditions which obtain in such locali- 
ties, are sufficiently different from mesophyte conditions, on 
the one hand, and true hydrophyte conditions on the other, to 
give the floras of swamps and marshes a character of their own. 

193. Comparing xerophytic plants with hydrophytes we find 
that in a number of particulars they present the opposite ex- 
tremes of structures. Thus the epidermis of xerophytes is 
extremely well developed, often consisting of several layers of 
cells and provided with a very thick cuticular wall on the sur- 
face. Stomata are less numerous. The plant as a whole is 
more compact, thus reducing the ratio of surface to volume. 
All these peculiarities result in decreased loss of water by 
transpiration and evaporation and are clearly an adaptation to 
scanty water supply. The massive form of these plants also 
affords space for the storage of water obtained from occasional 

194. Under semi-arid conditions there is sometimes another 
device employed for preventing the excessive loss of water, 
namely, the vertical or meridional position assumed by the 
leaves or leaf -like organs (phyllodes) and the consequent tem- 
pering of the force of the sun's rays. 

195. In those regions of the earth's surface which have 
alternately wet and dry seasons the vegetation also presents 
alternately mesophytic and xerophytic characters. This does 
not mean merely that at one season mesophytes are prominent 
and at another the xerophytes, but even the same plant alters 
in character with the seasons. 




196. There are certain parts of the earth's surface which are 
always destitute of vegetation because of the fact that the 

FIG. 40. A small fern, Polypodium, which grows as an epiphyte on the bark of 
trees. See next figure. XL 

surface waters are always frozen, a condition which renders 
vegetable life impossible. On the other hand, in the hottest 


parts of the earth vegetation flourishes, provided there is a 
sufficient supply of moisture. In fact, it is in equatorial regions 
that vegetation grows most luxuriantly. 

197. Between the two extremes of latitude, with the corre- 
sponding extremes of cold, and heat and absence and profusion 
of vegetation, there are regions which present an alternation of 
conditions with respect to temperature, from winter to summer. 

FIG. 41. Same as the preceding figure but photographed on the preceding 
day. The plant has the xerophytic habit of curling up during dry weather as in 
this figure. In wet weather the leaves expand. 

The change of seasons permits only of intermittent periods of 
growth, and this has affected most of the species indigenous 
to such regions to such an extent that the life processes succeed 
each other in a rhythmical manner, even though the conditions 
are temporarily altered. The lower orders of plants respond 
more directly to actual changes of the conditions, and they vary 
in character directly as the conditions vary, but the higher 



forms show a decided tendency to undergo their usual series 
of life processes and accompanying change of character, even 
though the seasonal changes of conditions fail to occur at the 
appropriate time. For example, the tropical plants present an ; 
expanse of leaf surface throughout the year, although the older ; 
leaves are continually falling, because new ones are as constantly 
developing. The deciduous perennials are characteristic of 
temperate latitudes, where there is alternately winter and sum- 
mer. In the latter case there is evidently a relation between 
the fall of the leaf and the seasons. But the leaves do not fall 
only after they have been killed by a frost. Rather, they die, 
and physiological connection with the plant body is cut off 
before the time for serious frosts arrives. Otherwise not only 
would the leaves be killed, but the plant itself might suffer 
serious injury. 

198. For the plant to retain its leaves during the snows of 
winter would also expose it to the danger of being overloaded 
and crushed by sheer weight of the snow. 

199. Another example of this principle of the independence 
of the plant of the direct conditions of its environment is found 
in the period of rest required by seeds and other reproductive 
bodies, such as bulbs, which normally remain quiescent during 
the winter and resume their growth with the recurrence of the 
warmth of spring. But the rest is taken whether or not the win- 
ter conditions supervene. 

200. Such adaptations of the plant are not responses to 
changing external conditions. The plant undergoes changes 
which anticipate the corresponding changes in the conditions. 
Such adaptations must be regarded as habitual responses. 

20 1. Some other adaptations of the plants to rigorous 
climates are, for example, such modifications of the leafy shoot 
as the rosette and the creeper. By hugging the earth plants 
of this type avoid the great exposure to cold which a freer 
method of growth would entail. 


202. Subterranean stems and reserve food stores generally 
ire devices for tiding over unfavorable seasons and permitting 
the plant to make the best of a short growing season. The 

FIG. 42. Cassia, the wild sensitive pea. 

mnual and biennial plant habits are also evidently adaptations 
:o seasonal changes, whether of temperature or moisture. 

203. The usual fall of temperature at night, which is often 
rery considerable, is a condition to which some plants apparently 


show a very special response. The usual day-time disposition 
of the leaves is such as would at night result in the greatest 
loss of heat by radiation. The leaves of many plants droop at 
night and thereby come into a position which greatly reduces 
the loss of heat by radiation. 

Latitude and Altitude 

204. The traveler in passing from the equator to the latitudes 
of perpetual snow in polar regions observes a gradual change in 
the character of the vegetation from the most luxuriant evergreen 
tropical forests to the scanty herbage of those high latitudes 
where during the few weeks of the brief summer, while the ground 
is bared of snow, a few specially hardy mosses, a few rapidly 
maturing annual and biennial herbs and still fewer shrubby 
perennials succeed in bringing their fruit to maturity. So 
also in ascending mountain slopes from the sea-level at the 
equator to the snow line on the higher peaks a similar series 
of changes in the character of the vegetation occurs with the 
degrees of altitude. In middle altitudes, as in middle latitudes, 
there is an intermediate condition of vegetation characterized 
by the grasses of the prairies and the deciduous perennials and 
coniferous evergreens of the forests. 


205. The adaptations of green plants to light conditions 
has been discussed at considerable length with reference to the 
disposition of the leaves. It remains to show that the adapta- 
tion extends also to the formation of palisade tissue and the 
arrangement of the chloroplasts within the cells of themesophyll. 
In order to determine whether the palisade tissue is the result 
of a response to light stimulus, the following experiment was 
performed. A developing leaf was artificially inverted so that 


what should have been the underside was brought uppermost 
and facing the sun. The result was that the palisade tissue was 
developed on the side toward the light, that is, on the mor- 
phological underside. 

FIG. 43. Leaves of the sensitive pea closed. In this case the leaves close when 
touched. No explanation for this habit is known. 

206. The position taken by the chloroplast in the cells is 
also determined by the intensity of the light. In bright light 



they are found to be crowded on the vertical walls of the cells, 
while in subdued light they are ranged on the horizontal 
walls, thus exposing themselves broadside to the light. In this 
way the chloroplasts to a considerable degree control the 
light relation of the plant. 

207. Plants might be classified with reference to the light 
conditions of their habitat. Many species grow only in shaded 
situations, while others seek the brightest light. Under the 
vertical rays of a tropical sun the light is much more intense than 
it is in higher latitudes. Consequently it penetrates the 
foliage of the taller forest vegetation with sufficient intensity 
to permit also of a vigorous undergrowth. The result is that 
other things being equal the intensity of the light in the tropics 
permits a denser growth of vegetation than could exist in higher 

208. The deep sea is known to be practically destitute of 
vegetation, although the conditions, except for darkness, are 
probably favorable. Along shore and on the surface of the sea 
there is an abundance of green and brown sea- weed vegetation, 
and farther down, on comparatively shallow bottom, the red 
sea-weeds are found. It has been suggested that the red and 
brown pigments of the red and brown sea- weeds have some 
significance with reference to the light relation. 


209. Every farmer is familiar with the fact that the dis- 
tribution of plants is largely determined by the nature of the 
mineral constituents of the soil. Thus limestone regions are 
better adapted for the cultivation of certain crops than are soils 
derived from sandstones or shales. So also other plants do well 
only on sands or clays. Analysis of the soils, however, shows 
that all the mineral substances necessary to any plant are 
present in sufficient quantities in any soil. It is also known that 


plants differ greatly in regard to their ability to thrive on soil 
containing little organic matter, and it is probable that this is 
really the determining cause of this apparent soil relation. 
Soils of unlike mineral constitution do not retain the organic 
matter to the same degree and it is therefore probably the 
amount of humus present in the soil that determines its adapta- 
bility to any given plant. 

210. Occasionally soils are too unstable to permit vegetation 
to secure a foothold. This is notably true of the shifting sand 
dunes of many windward coasts and in sandy deserts. There 
are a few plants, however, which are enabled to maintain their 
position in such soil by virtue of rapid and deep rooting or, 
better still, by means of stolons or runners which enable the 
individual stocks to cling to each other and finally, forming a 
felted carpet, protect the sand from the wind and hold it in 

Relation of Plants to Each Other 

211. We have heretofore spoken of the green plants as being 
independent in the sense of deriving their sustenance directly 
from inorganic matter. This might be regarded as quite 
generally true wherever the plant is provided with soluble 
nitrogen compounds. However, these salts are by no means 
everywhere present in the soil, and under such circumstances 
green plants become dependent upon certain fungi, as we shall 
presently see. 

212. Plants may be dependent upon other plants in a great 
variety of ways, and in varying degrees. The climbers, for 
example, get only mechanical support from their stouter neigh- 
bors. Some cling to their support by means of aerial rootlets, 
which penetrate the outer layers of the bark of the host. Others 
spread their long slender tendrils, which, on contact with a solid 
object, coil around it and then draw the stem of the plant close 


to the support by the contracting spirals. Tendrils in some 
instances eiid in adhesive discs by which the plant is enabled to 
cling to a smooth plane surface. Lastly, the twining climbers, 
swaying their growing tips in a spiral around the axis of support, 
coil themselves bodily about their host. In neither of these 
cases does the climber obtain any nourishment from the host, 

FIG. 44. The trumpet vine, a climber. 

although the latter may be seriously handicapped or even 
finally destroyed through shading by its vigorous yet dependent 

213. Epiphytes constitute another class of plants which 
depend upon others for mechanical support. They have no 


connection with the soil and obtain their necessary supply of 
moisture from the humid atmosphere, from the moist bark of 
the host, or from the sponge of vegetable detritus which accu- 
mulates about the base of the plant. The epiphyte by its habit 
merely obtains advantageous exposure to light. 

FIG. 45. An epiphyte, Tillandsia, hanging from the branches of trees, 
landsia is a flowering plant but is erroneously called "gray moss." 


214. Saprophytes are plants found growing only on humus 
or other decaying organic matter. They are of great impor- 
tance in the economy of nature because of their share in the 
process of decomposition and decay. They contain no chloro- 


FIG. 46. Puff-balls, Lycoperdon, a saprophyte. XL 

FIG. 47. The truffle, Tuber brumale; a saprophyte which grows underground, 




phyll, have no power of photosynthesis and consequently bear 
no necessary light relation. The group includes chiefly fungi, 
but there are not a few flowering plants which have degenerated 
to the condition of saprophytes. 

215. What has been said of saprophytes might be repeated 
for the group called parasites; excepting this, that they live in 

FIG. 48. Section ol a lichen. Near the upper surface are groups of rounded 
cells (shaded). These are algal cells, arranged in groups by fission. The re- 
maining parts are formed by the filaments of the fungus. (From Sayre after 

or upon the tissues of living organisms and the result of their 
activity is called disease. Most infectious diseases of both 
animals and plants are to be ascribed to this class of organisms. 
A few chlorophyll-bearing plants have a parasitic habit and 
hence are called partial parasites. Facultative parasites may 
live either as true parasites or as saprophytes, while obligate 
parasites can exist only as parasites. 



2 1 6. Symbionts are organisms which are associated with 
other plants or animals for the advantage of one or both, but 
without serious detriment to either. Some forms are found 
normally only under such relationship. Lichens are a symbiotic 
combination of a fungus and an alga. A relation of this kind 
also exists between certain Spermatophytes and some of the 

FIG. 49. The sun-dew, Drosera. One leaf is shown, with the glandular hairs 
by which small insects are caught. X5/3- 

lower fungi. The mycorhiza, for example, consist of a filamen- 
tous fungus attached to the roots of seed plants, for which they 
seem to serve to some extent the office of absorption, and prob- 
ably receive some compensation in return. 

217. The most important of this class of plant relationship 


is that which exists between bacteria and seed plants. Certain 
bacteria live in the soil and have the power of assimilating the 
free nitrogen of the air, of breaking up ammonia compounds, or 
of oxidizing nitrites into nitrates and thus bringing the nitrogen 
compounds into a form available for green plants. Such nitro- 
gen bacteria accumulate in masses on the roots of leguminous 

FIG. 50. The trap of the Venus fly- trap, half closed. X2. 

plants, presumably finding there conditions favorable to their 
own development and certainly enabling the associated seed 
plant to thrive in soil which would otherwise be too poor in 
available nitrogenous compounds to support the plant. 

Carnivorous Plants 

2 1 8. A considerable variety of unrelated plants have acquired 
the power in one form or other of capturing small animals and 

9 6 


by a process analogous to digestion and absorption securing 
in this way the requisite nitrogenous matter. Such carnivorous 
plants are found in soils or situations which are in an unusual 
degree devoid of nitrogenous compounds of all kinds. 

Physiographic Relations 

219. Finally we may note briefly the physiographic relations 
of plants. From geological evidence we know that vegetation 

FIG. 51. The Venus fly-trap closed, showing interlocking teeth. 

has existed upon the earth for vast ages and has undergone 
continual changes with the lapse of time. Long before the 
advent of man, vegetation flourished as it has not done since, 
and in certain regions of the earth's surface the remains of 
those early forests accumulated to such an extent as to form 



FIG. 52. Venus fly-trap. Teeth unlocked but trap still closed. The dark 
shadow is cast by the bodies of two house flies caught by the trap. 

FIG. 53. Trumpets, one of the pitcher plants (Sarracenia flava). 


the many deposits of coal, covering in the aggregate thousands 
of square miles of the earth's surface and varying in thickness 
from a few inches to hundreds of feet. 

FIG. 54. Sarracenia minor. 

220. Practically all the coal-forming plants have long since 
become extinct. They belonged chiefly to the Cryptogams, 
and very few of the higher plants existed at the time. The 



species of Spermatophytes living to-day have all appeared on 
the earth in comparatively recent times. 

221. Plant remains, therefore, in the form of coal, constitute 
a very considerable part of the earth's crust and have largely 
determined the modern conditions of human activity. But 

FIG. 55. Sarracenia purpurea, in bloom. In this species the leaves are shaped 
like a pitcher. Xi/3- 

vegetation is at the present time an important agency in both 
constructively and destructively modifying the physiographic 
features of the earth. 

222. All forms of vegetation assist in breaking up rocks and 
dissolving minerals and thus contributing greatly to the general 
process of weathering or decay of rocks. Vegetation is also 


a conservative force in protecting the surface of the land from 
erosion by holding the soil in position and breaking the force 
of the rush of surface waters, which rapidly wears away naked 
or unprotected soils. It is even a constructive agency in many 
cases, as, for example, in the formation of deposits of coal, peat, 
iron ore and other minerals, and in filling up swamps, ponds, 

FIG. 56. A solid-rock surface covered with lichens, mosses and ferns. 

lakes and sluggish streams with the wash from the land, and 
even in places building out the land into the" shallow open sea. 
223. The general climatic conditions of a region may also 
be modified by changes in the vegetation so as to determine 
very materially the social and economic conditions of man. The 
deforestation of a region has in more than one instance resulted 
in the practical destruction of a highly developed civilization. 



224. BRANCH I. T hallo phytes. To this division of plants 
belong all of the lowest eleven classes. No single positive 
character is common to all. The plant body is never differ- 
entiated into true roots, stems and leaves. Classes 3-8 con- 
tain chlorophyll and are called algae. Classes 9-1 1 are devoid 
of chlorophyll and are called fungi. 

FIG. 57. A slime mold creeping over dead grass. X i. 

225. Class i. Myxomycetes. This group is sometimes 
classed with animals under the name mycetozoa. The organism 
is saprophytic and is found creeping about on moist organic 
matter, as on the vegetable mold under trees. In the active 
condition a slime-mold plasmodium might be likened to a 
gigantic amoeba, often several inches in diameter, with numer- 



ous nuclei. It creeps about by an amoeboid motion for a time 
but then comes to rest. The central part of the mass becomes 
transformed into spores while the superficial parts form a 
peridium or spore case which opens when the spores are ripe 
and permits them to scatter as a dry brown powder. When 
the spore germinates an active swarmspore with a flagellum 
emerges. This greatly resembles a flagellate. After a time 
the flagellum is lost and the organism assumes an amoeboid 
(Myxamceba) condition. By the fusion of a number of these 
myxamcebae the plasmodium is formed but in this fusion the 
nuclei are not concerned. There is nothing resembling a 
sexual method of reproduction. The vast number of spores 
produced results from the division of the nuclei of the plas- 
modium. A few slime-molds are parasitic in plants. There 
is no chlorophyll and the reason for placing these organisms 
under the plants rests on the very unanimal-like condition of 
the organism in the " fruiting" stage. 

226. Class 2. Schizophyta. The organisms belonging to 
this class are very simple in structure. There is no well-defined 
nucleus and the cells are usually very small. There is no 
sexual reproduction and multiplication takes place by fission. 
The cells are either free or adhere in chains, plates or masses 
held together by the gelatinous cell wall. 

227. Order i. Bacteria. The bacteria are the smallest organisms. 
Many forms are so minute that even with the highest power of the micro- 
scope they appear as little more than a point. There is a cell membrane 
but no nucleus. Certain granules scattered in the cytoplasm stain like 
chromatin and are therefore supposed to represent the nucleus. The 
bacteria contain no cholorophyll, consequently most are saprophytic or 
parasitic. But some forms are holophytic. Some forms have one or 
more cilia and are motile, others are motionless and may be embedded in 
a jelly formed by the swelling of the cell membrane. There is compara- 
tively little variety in form because of the simplicity of structure. Never- 
theless there are many species and these can be distinguished by their 
physiological characters. The half dozen type forms which commonly 


occur have been given special names as follows: A coccus is spherical 
in form, a bacterium or bacillus is short rod shaped, a bacillus slightly 
bent is a vibrio, while one more strongly curved is a spirillum; straight 
thread-like forms are called leptothrix and corkscrew forms are spirochaete. 

FIG. 58. Bacilli of various forms. (From Williams.) 

228. After division the cells may adhere in chains or become free. When 
the cell walls become gelatinous the cells adhere in a large mass which is 
known as a zooglea. Spores are formed by the contraction of a part of 
the protoplasm into a dense mass which then surrounds itself with a cell 

* ->' 



FIG. 59. Spirilla of various forms. (From Williams.) 

membrane. Because of their mode of formation these are called endo- 
spores. They are highly resistant. 

229. The physiological differences between bacteria are very great. 
This is evident in the substances which they excrete and the effect pro- 
duced by these excretions on the surrounding medium. A number of 


FIG. 60. Staphylococci. Streptococci. Diplococci. Tetrads. Sarcinae. 
(From Williams.) 

examples will be mentioned. Closteridium butyricum and many other 
bacteria thrive where there is no free oxygen. This is possible because 
they have the power of decomposing substances containing oxygen. On 
the other hand, Bacillus aceti, and others, cause the combination of alcohol 
and oxygen to form acetic acid (vinegar) . The nitrite bacteria in the soil 
oxidize ammonia into nitrites and the nitrate bacteria continue the oxi- 


dation into nitrates. These bacteria and others are holophy tic, assimilating 
carbon dioxide like green plants. Beggiatoa alba grows in water where 
there is sulphuretted hydrogen, H 2 S, formed by the decomposition of 
organic matter. The bacterium causes the oxidation of the H 2 S whereby 
the sulphur is set free and deposited in the cell in small granules. Lepto- 
thrix ochraea oxidizes iron carbonate into iron oxide (iron ore) which 
is likewise deposited in the cell. 

230. Through the ferments formed by many bacteria sugar is formed 
from starch arid the sugar is then split into alcohol and carbon dioxide, 
C6Hi2O6=2C 2 H 6 0+2C02. This is the common type of fermentation 
which takes place in the making of bread, wine, beer and other alcoholic 

FIG. 61. Staphylococcus aureus. (Williams.) 

fluids. Another common type of fermentation is produced by the Bacillus 
vulgaris and other bacteria. The process here is ordinarily spoken of as 
decay. Nitrogenous substances, like flesh, are decomposed and, among 
other products, sulphuretted hydrogen is set free. It is this gas which 
produces the evil odor so characteristic of this type of bacterial activity. 

23 1. Among the parasitic bacteria are some which cause little or no harm 
to the host. Some may even be useful, as when those inhabiting the 
digestive tract assist in the process of digestion (Bacillus coli communis). 
But again others may be the cause of the most malignant and contagious 
diseases. Species of Streptococcus and Staphylococcus are generally 


the causes of local eruptions, such as boils, ulcers, gangrene, etc. Bacillus 
typhi in the digestive tract causes acute inflammation typhoid fever. 
Bacillus pneumoniae and the B. diphtheriae on the mucous epithelium of 
the pharynx and adjacent cavities, and B. tuberculosis in the lungs and 
on other serous membranes of the body are well known. B. tetani in 
the blood is the cause of lock-jaw. B. anthracis causes a disease fatal to 
cattle and occasionally to man. Asiatic cholera, leprosy and many dis- 
eases of domestic animals, such as chicken cholera, foot rot, black leg, etc., 
are bacterial. 

232. Order 2 . Cyanophycea (Schizophycea) . The Cyanophyceae are also 
called blue-green algae because of the presence of a blue pigment (Phyco- 
cyanin) in addition to chlorophyll. These plants are found only in water 
or on moist surfaces. They multiply by fission like the bacteria and the 
cells adhere in threads or are enclosed in masses of jelly formed by the 
swollen cell membranes. The nucleus usually consists of scattered 
chromatin granules. 

233. Class 3. Diatomeae. The Diatoms are a large group. 
They are also unicellular and the cells separate completely 
though they may adhere in chains or be attached by a common 
stalk. The cells are usually bilaterally symmetrical and the 
cell wall consists of a silicious capsule of two parts which fit 
into each other. The surface of the capsule is often very 
elaborately ornamented. There is a single central nucleus 
and one or more large lobed chromatophores containing a 
brownish-yellow pigment in addition to a substance similar 
to chlorophyll. Multiplication takes place asexually by fission 
and also by conjugation. 

234. Class 4. Conjugatae. The Conjugate are unicellular, 
though the cells may be connected in filaments. The cell has 
a single nucleus, one or more chlorophyll green chromatophores 
of a complicated form and one or more pyrenoids. Sexual 
reproduction through the union of two non-motile gametes 
(conjugation) to form a zygospore, is characteristic of the group. 

235. Order i. The Desmidiacece are single cells which consist of two 
symmetrical halves often joined by a narrower portion like a dumb-bell, 
the cells are frequently very bizarre in form. The nucleus lies in the 


narrower middle part of the cell and the chromatophore is symmetrically 

236. Order 2. The Zygnemacece are always filamentous and the cells 
are cylindrical. In conjugation the entire contents of the cells is involved. 

237. Order 3. The Mesocarpacece are similar to the foregoing but 
only a part of the cell contents is concerned in conjugation. 

238. Class 5. Chlorophyceae. The Chlorophyceae are a 
large group of fresh water and marine chlorophyll green algae. 
They reproduce asexually by the formation of pear-shaped zoo- 
spores which have two or four flagellae. Sexual reproduction 
usually consists in the conjugation of similar zoospores but 
there is often a differentiation of gametes into eggs and sperms. 

239. Order i. The Volvocales are motile throughout life. They are 
usually single and resemble the green flagellates, but some forms adhere 
by their gelatinous walls and form swimming colonies. The cell has a 
single nucleus and a chromatophore. 

240. Order 2. The Protoccocales are similar to the Volvocales but are 
only motile in the zoospore stage. 

241. Order 3. The Ulotrichales are usually simple or branched fila- 
mentous forms but some marine species form flat ribbons of two layers of 
cells. The cells are uninuclear and have usually one chloroplast. 

242. Order 4. The Siphonocladiales are also filamentous forms, usually 
much branched. The filaments are composed of large multinuclear cells 
with one or more chloroplasts. 

243. Order 5. The Siphonales consist of a branching tubular thallus 
with few or no cross walls in the vegetative condition. The protoplasmic 
substance is therefore continuous, with numerous nuclei and chloroplasts. 

244. Class 6. Characeae. The Characeae are fresh-water 
algae of rather complicated structure and with highly differ- 
entiated gametes and gametangia. The principal axis of the 
thallus consists of alternately long and short tubular cells form- 
ing nodes and internodes. A whorl of branches occurs at each 
node and the branches resemble the main axis in structure. 
Short branches of a second order may also occur and in the 
axils of these are found the oogonia and antheridia. The first 

ALGAE 107 

consist of an egg cell surrounded by a wall composed of five 
spirally wound branches. The antheridium is a complicated 
spherical structure with a wall of eight cells and containing a 
large number of spermatozoids each provided with two flagellse. 
No swarm spores are produced. 

245. Class 7. Phaeophyceae. The Phaeophyceae are the 
brown sea weeds. Only a few small forms are found in fresh 
water. Among the marine forms, however, are the largest of 
all Cryptogams. Macrocystis pyrifera is said to attain a 
length of over 200 feet. The plants are usually attached to 
rocks by means of a hold-fast organ. No general statement 
can be made concerning the form but in the larger species the 
thallus is usually flattened and often forms broad sheets. The 
cells are uninuclear and contain a number of chromatophores 
which contain a brown pigment, phycophaein. 

246. Order i . The Phceosporecz reproduce asexually by means of swarm 
spores produced in large numbers in "unilocular" sporangia. Sexual 
reproduction occurs also through the conjugation of motile gametes 
developed in multilocular sporangia (gametangia) one gamete being 
developed from each cell of the sporangium. 

247. Order 2. The Cyclosporea are farther advanced sexually. There 
is a marked differentiation of egg and sperm. In one family, Dictyo- 
taceae, asexual aplanospores are also produced but in the Fucaceae there is 
no asexual reproduction. 

248. Class 8. Rhodophyceae. The Rhodophyceae or Flor- 
ideae are the red sea weeds. A few forms occur in fresh water. 
The red sea weeds are small as compared with the brown. The 
form of the thallus is most often a bushy mass of branching 
delicate filaments or of thin sheets. Some species are encrusted 
with calcium carbonate. The color is due to a red pigment, 
phycoerythrin, found in the chromatophores in addition to the 
green. Asexual reproduction takes place through non-motile 
tetraspores, which are produced in groups of four on the surface 
of the thallus. The sexual reproduction is peculiar. The male 


cells are minute non-motile "spermatia," cut off from the tips 
of certain branches. The female branch, carpogonium, is ter- 
minated by a slender filament, the trichogyne. When a sper- 
matium comes in contact with this trichogyne a fusion takes 
place which effects a fertilization. The result is that by a more 
or less indirect process spores, carpospores, are developed 
farther down on the branch. 

249. Class 9. Phycomycetes. The Phycomycetes are fungi, 
that is, they contain no chlorophyll and are therefore sapro- 
phytic or parasitic in habit. But in many other respects they 
resemble algae, especially the Siphonales. The thallus is tubu- 
lar with few or no cross walls dividing it into cells, and the proto- 
plast contains numerous nuclei. Spores are formed asexually 
by the division of the protoplasmic contents of a sporangium. 
These spores are motile in aquatic forms but those of terrestrial 
forms are simple rounded cells which are scattered like dust. 
In some genera a kind of spore, conidium, is formed by the 
cutting off of a cell from the tip of a filament (hypha) . 

2 50. Order i . The Oomycetes reproduce sexually by means of sperm and 
egg cells. In some cases the sperm cells are motile spermatozoids which 
are set free from an antheridium, make their way to the oogonium and 
fuse with the egg cell producing an oospore. In most cases, however, 
the antheridium forms a tube which grows toward and into the oogonium. 
In this case the sperm cells are not provided with flagellae. They pass 
through the tube of the antheridium directly into the oogonium and there 
reach the egg cell. The asexual spores are swarm spores. The oomycetes 
grow as saprophytes in fresh water or as parasites in plants and, occa- 
sionally, animals. 

251. Order 2. The Zygomycetes reproduce sexually by the fusion of the 
contents of two similar gametangia. The resulting body is called a zygo- 
spore. The asexual spores are produced either in sporangia or as conidia. 
The Zygomycetes are terrestrial and grow as saprophytes on vegetable or 
animal matter or as parasites in insects. The black mold on bread, etc., 
is a familiar example. 

252. Class 10. Basidiomycetes. The Basidiomycetes are 
distinguished by the club-shaped basidium upon which four 


spores are produced by budding. The basidium is a terminal 
hypha and is either unicellular and bears the four spores at its 
free end or else it is divided into four cells each one of which 
bears one spore. 

Rudimentary sexual organs are found but there is usually no 
sexual process connected with the formation of spores. The 
basidia are usually grouped and borne on the surface, or within 
special fruiting bodies. Besides the basidiospores there may be 
also one or more other kinds of spores formed by the same 

253. Order i. Hemibasidiales are parasitic on plants. The black 
corn-smut is a familiar example. A short hypha which is supposed to 
represent the basidium is developed directly from the thick-walled brand 
spores. The mycelium developed from the basidiospores ramifies through 
the tissues of the host and ultimately produces large masses of brand spores. 

254. Order 2. Protobasidiomycetes have a basidium divided into four 
cells each of which bears a spore. The "rusts'.' are the most important 
members of this group. See p. 364 for the life history of the wheat rust. 
There are also some saprophytic forms which have a gelatinous thallus. 

255. Order 3. The Autobasidiomycetes have the basidium undivided 
but bearing four sterigmata with one basidiospore on each. The group is 
a very large one. In most cases the basidia are arranged in a well-defined 
layer called the hymenium and this is borne on a fruiting body of very 
definite form. The hymenium may form a single flat surface, or it may 
be variously folded into numerous tooth-like of finger-like projections or 
parallel plates. In other groups again it lines the walls of slender tubes 
or of numerous closed chambers. To this order belong the mushrooms and 

256. Class 1 1 . Ascomycetes. The Ascomycetes are charac- 
terized by the sack-like sporangium, the ascus. This is formed 
from the terminal cell of a hypha, the two nuclei of which fuse 
and then divide, usually three times so that eight spores are 
formed. Not all of the protoplasm of the ascus is used in the 
formation of the spores. The asci are usually clustered in 
characteristic fruiting bodies. Sexual reproduction by oogonia 
has been observed in a few cases. 


257. Order i. The Peris poreacea comprise the mildews and the blue 
molds. In this group the asci are completely enclosed in a minute spher- 
ical fruiting body, the perithecium. The mildews are parasitic on the 
leaves of higher plants as exemplified by the common grape and lilac mil- 
dews. The blue molds are saprophytic on decaying fruits, preserves, 
bread, leather, etc., and are usually readily distinguished from the black 
molds (Phycomycetes) by the color and by the conidia. 

258. Order 2. In the Discomycetes the asci are borne in concave disk- 
shaped fruiting surfaces (apothecia). The species of this order are very 
common and are usually saprophytic. They are most frequently found 
on decaying wood. The edible morel (Morchella) belongs to this order. 

259. Order 3. In the Pyrenomycetes the perithecium is flask shaped 
with a pore through which the spores escape. In the mature condition 
the fruiting bodies are usually black. The Pyrenomycetes are in part 
parasitic; some on other plants as the black knot of plumb trees or the 
ergot of rye and some in the bodies of insect larvae. Many forms are 
saprophytic on bark, decaying wood, etc. 

260. Order 4. The Tuber acea are saprophytic underground in forest 
humus. The perithecia are large spherical bodies without opening. 
Some forms are edible. 

261. Order 5. The Exoasci are parasitic on trees and stimulate the 
tissues of the host to abnormal growth thus forming in certain cases 
"witches' brooms." 

262. Order 6. The Saccharomycetes (yeasts) are microscopic unicellular, 
saprophytic fungi. They are important as the chief agents in the various 
fermentation processes connected with the making of bread, wine, beer 
and other alcoholic liquors. Reproduction takes place by budding 
(conidia) and under favorable conditions spores are formed endogenously 
within a cell (ascus). 

263. Lichens. The lichens are one of the most common and most widely 
distributed types of vegetation. They may be found on almost any kind 
of stable surface, on rocks, tree trunks, or on the surface of the earth. 
They are often confused with mosses but are readily distinguishable. 
The color is usually gray or brown but never chlorophyl green. The 
fruiting surfaces are frequently brilliantly colored. The form of the plant 
is thalloid, never differentiated into true stem and leaf. Serious objection 
may be made to ranking them as a class because a lichen is in reality only 
a symbiotic combination of an alga and a fungus, either of which may be 
grown independently of the other. The algae concerned would by them- 
selves be classed as Cyanophyceae or as Chlorophyceae of the simpler forms. 


The fungi are usually Ascomycetes but a few Basidiomycetes also occur 
as lichens. The fungous filaments wind about the algal cells and usually 
form a firm superficial protective tissue. The specific characteristics of 
a lichen depend upon both the alga and the fungus components but the 
fruiting surface of the lichen is purely fungal. (See page 362.) 

264. BRANCH II. Bryophyta. The Bryophyta include the 
liverworts and mosses. They are distinguished from the Thal- 
lophytes by the structure of the gametangia. The sperm cells 
are provided with two long flagellae and they are formed in large 
numbers within an oval capsule, the antheridium, whose walls 
are composed of a single layer of cells. The single egg cell is 
contained in the lower part of a flask-shaped gametangium, 
called archegonium. This is also formed by a single layer of 
cells. The upper portion, or neck, of the archegonium contains 
an axial row of cells (canal cells) which disintegrate and 
form a slime through which the spermatozoids make their way 
to the egg. A " ventral canal cell" cut off from the egg cell 
at the lower end of the canal cells also distintegrates with the 
canal cells. 

265. The fertilized egg immediately begins development. 
There is ultimately formed an organism (sporophyte) which 
produces spores asexually. From these spores there then 
develops an organism (gametophyte) which bears the gametan- 
gia. In this way a regular alternation of sexual and asexual 
generations occurs. 

266. The Bryophyta are called Archegoniates from the very 
characteristic female gametangium. They are always holo- 
phytic and are chlorophyll green. 

267. Class i. Hepaticae. The liverworts are usually thalloid 
or if there is a differentiation into stem and leaves the latter 
are arranged dorso-ventrally. The thallus branches dichoto- 
mously and is attached to the substratum by rhizoids, i. e., tubu- 
lar, hair-like cells which serve as organs for the absorption of 
moisture. The spore capsules usually contain sterile elongated 


cells (elaters) which serve to scatter the spores by hygro- 
scopic movements. 

268. Order i. The Ricciacea are aquatic or semi-aquatic thalloid forms. 
The sporophyte remains completely enclosed within the archegonium wall 
and embedded in the gametophyte thallus. It consists merely of a spher- 
ical capsule filled with spores. 

269. Order 2. The Marchantiacea are larger and more complex in 
structure. The thallus is perforated by pores on the upper side, which 
open into air chambers. Surrounding and projecting into these chambers 
are the green assimilatory cells. The deeper lying cells are larger, 
contain little or no chlorophyll and serve for water storage and con- 
duction. The antheridia and archegonia are borne on stalked re- 
ceptacles. The sporophyte is a stalked capsule which remains attached 
to the receptacle but projects beyond the old archegonium wall. 

270. Order 3. The Anthocerotaceae are a smaller group. The game- 
tophyte is irregular thalloid. Its cells contain each a single chloroplast. 
The archegonia are embedded in the thallus. The sporophyte projects 
beyond the thallus because of its greatly elongated form, but is not stalked. 
The capsule splits longitudinally and there is a slender axial "columella" 
of sterile tissue. 

271. Order 4. The Jungermanniacece are usually differentiated into 
a stem and dorso- vent rally arranged leaves one cell layer thick. The 
capsule is long stalked and usually opens longitudinally by four valves. 
There is no columella. 

272. Order 5. The Calobryacece are represented only by two exotic 

273. Class 2. Musci. The moss plant is differentiated 
into stem and leaves. There are no true roots but at the base 
of the stem are found branching rhizoids by which water is 
absorbed. The stem sometimes contains an axial strand of 
elongated cells which serve for the conduction of fluids, but 
there are no true nbro-vascular bundles. The leaves are usually 
one cell layer thick and are arranged spirally on the stem. 

274. The archegonia and antheridia are borne at the apex 
of the stem or on lateral branches. The sporophyte is stalked 
and remains attached to the gametophyte by a prolongation 


of the stalk known as the foot. The capsule contains a central 
axis of sterile tissue, the columella. 

275. When the spores germinate a branching green thread 
(protonema) like an alga is developed. From this the moss 
plants are formed by budding. By this protonema the mosses 
may be distinguished from the liverworts. The mosses are 
always holophytic and chlorophyll green. 

276. Order i. The Sphagnacece are the swamp mosses. They grow 
continually upward while dying away at the base. The capsule is short 
stalked and the foot is broad. A pseudopodium is formed by the elonga- 
tion of the branch below the foot. The archegonium wall breaks and its 
fragments remain at the base of the capsule. The capsule opens by a lid. 
The columella is hemispherical. There is only one genus, Sphagnum, 
with many species. 

277. Order 2. The Andreacea are usually found in small clusters on 
rocks. The capsule is short stalked with a broad foot and is elevated by 
a pseudopodium. The archegonium wall breaks around the base and 
remains on the capsule like a cap (calyptra). The capsule opens by four 
longitudinal slits. There is only one genus, Andrea. 

278. Order 3. The Phascacece are small, simple mosses with persistent 
protonema, and a short stalked capsule which does not open. The spores 
are set free only by the disintegration of the capsule. 

279. Order 4. The Bryina comprise most of the common mosses. In 
these the capsule is long stalked (seta) with a foot. The archegonium 
forms a calyptra. The capsule opens by a lid (operculum). On removal 
of the operculum the edge of the opening of the capsule is seen to be pro- 
vided with a fringe, the peristome, which by hygroscopic movements 
assists in the scattering of the spores. 

280. BRANCH III. Pteridophyta. The ferns are differen- 
tiated into true root, stem and leaf. Conducting and sup- 
porting tissues in the form of fibre-vascular bundles occur, as in 
the higher plants. The ferns are like the mosses in regard to the 
structure of the gametangia and are hence archegoniata. Like 
the mosses they also have a distinct alternation of generations. 
The gametophyte, however, is reduced to an inconspicuous 
thalloid structure (prothallium) or still farther to a minute 



cluster of colorless cells. The sporophyte, on the other hand 
is much better developed and constitutes the leafy plant. 

281. Class i. Filicinae. The Filicinae are the true ferns, 
a large group of plants of moderate size. A few tropical " tree 
ferns" attain the size of a small tree but the more familiar 
forms have only an underground stem (root-stock) from which 
the leaves (fronds) rise on long petioles (stipes) to a height of 
from i to 5 feet. All ferns are holophytic. Many species, 
especially tropical ones, are epiphytic. 

282. The gametophyte of the fern is a small green thalloid 
structure (prothallium) which lies flat on the ground. (Or 
colorless, saprophytic and underground, Order i.) Embedded 
in its tissues are the antheridia and archegonia. The anthero- 
zoids are spiral bodies with a tuft of cilia at one end. The 
fertilized egg cell divides into four segments from which are 
developed root, stem, leaf and foot respectively. The foot is 
an organ by which the developing plant retains connection 
with the prothallus for some time. The prothallus finally 
disintegrates and the plantlet becomes independent. The 
plant with the root, stem and leaves is the sporophyte. The 
spores are developed in sporangia on the under surface of the 
leaves. Sometimes the spore-bearing portion of a leaf is espe- 
cially modified, or again the spores are only borne on certain 
leaves which then are completely modified (sporophylls) . 

283. Order i. The Ophioglossacea. or adder-tongue ferns, are a small 
group of slow growing and rather inconspicuous plants. The gametophyte 
is a small, saprophytic, underground thallus. The leaf is partly differ- 
entiated into sporophyll. 

284. Order 2. The Marattiacece are tropical ferns of large size. The 
prothallus is a green thallus resembling a liverwort. The sporangia are 
grouped in sori on the under surface of the foliage leaves. 

285. Order 3. The Filices are the order to which most of our common 
ferns belong. Most of the tropical tree ferns also belong to this order. 
The order comprises many genera and species. Many are epiphytic. 
The gametophyte is usually a small, green, liverwort-like prothallus which 


bears antheridia and archegonia on its under surface embedded in its 
tissue. The sporophyte usually bears the spores on the under surface 
of undifferentiated leaves. The sporangia are stalked and grouped in 
clusters (sori) . The sori are often covered by a scale (indusium) . 

286. Order 4. The Hydropteridece or water-ferns, are a small group of 
plants which bear little resemblance to common ferns. Some grow in 
the mud, partly submerged, others float on the surface of the water. 
They are of special biological interest because the sporophyte bears two 
kinds of spores, small " microspores " and large "megaspores." The 
microspores develop a very simple prothallus consisting of only a few 
colorless cells, and a few antheridia. The megaspores develop a slightly 
larger prothallus which, however, only projects slightly beyond the broken 
sporangium wall. (A megasporangium produces only one megaspore.) 
A few archegonia are formed in the prothallus but only one egg cell devel- 
ops. The microspores therefore develop male gametophytes and the mega- 
spores female. 

287. Class 2. Equisetinae. This class contains only one 
genus, Equisetum, the common scouring rush or "horse-tail." 
These plants have an underground stem from which the erect 
fruiting and vegetative stems rise each season. The stem is 
fluted and jointed and green since there are no foliage leaves. 
The scale leaves sheathe the stem at the nodes. The fruiting 
stems are usually simple while the vegetative stems bear whorls 
of branches at the nodes. The epidermis is encrusted with 
silica which gives the stem a harsh feel and lends the name 
scouring rush. The fruiting stems bear a conical spike of 
umbrella-shaped sporophylls. The spores are all of one kind 
and are each provided with four ribbon-like hygroscopic appen- 
dages (elaters) by which the spores are scattered. The spores 
give rise to a branching prothallus which is usually unisexual. 

288. Class 3. Lycopodinae. The Lycopodinae are small 
plants with some affinities to the ferns but of very different 
appearance. The sporophylls each bear a single sporangium. 

289. Order i. The Lycopodiacece are the lycopodiums, "trailing cedar" 
or "ground pine." The stem is usually trailing, with short ascending 
branches. Branching of stem and roots is dichotomous. The leaves are 


broad awl shaped and thickly set. The sporophylls are usually borne 
in a spike with the sporangia on the upper side of the scale-like leaves. 
The spores are all of one kind. The gametophyte is bisexual. It is a 
club-shaped saprophytic organism in some cases, in others it forms a flat 
green thallus. The spermatozoids are provided with two flagellae. The 
embyro develops a suspensor as in Selaginella. 

290. Order 2. Selaginellacea also all belong to one genus, Selaginella. 
Most of the species are tropical but a few delicate moss-like forms are 
found in our forests. The plant resembles lycopodium somewhat but 
the arrangement of the leaves is usually dorso-ventral. There are fre- 
quently two dorsal rows of very small leaves and two ventral rows of larger 
ones. The sporangia are borne in the axils of leaves near the tip of a 
branch. There are two kinds of spores found in the same spike. Some 
sporangia contain four megaspores, others numerous microspores. The 
microspores develop into a prothallium of one cell and an antheridium 
of eight cells within which a number of spermatozoids, each with two 
flagellae, are formed. The megaspore develops a small colorless prothal- 
lium in which a few archegonia are formed. Only one or two of the arche- 
gonia are fertilized. The embryo develops an appendage, the suspensor, 
which consists of a row of cells, by which the embryo is pushed down into 
the nourishing prothallium. 

291. Order 3. The Isoetacece consist of the single genus, Isoetes. The 
plants are small, with long needle-shaped leaves arranged in a rosette 
around a short erect stem. The plants are found submerged in water 
or in wet soil. The sporangia are borne on the inner surface of the leaves, 
at the base. The outer leaves bear megasporangia, the inner ones micro- 
sporangia. The spermatozoids are spiral and have a tuft of cilia like 
those of the ferns. The embryo has no suspensor. 

292. BRANCH IV. Spermatophytes. The three branches of 
the vegetable kingdom already described are together called 
Cryptogams and in distinction to them all the higher forms are 
called Phanerogams. The latter are in general more highly 
developed, but the distinguishing character is the seed, like 
which nothing is found among the Cryptogams. In the Phan- 
erogams the female gamete is developed within the megaspore 
wall and the egg cell is fertilized and develops an embryo while 
the megaspore is still within the sporangium and attached to 
the parent sporophyte. After the embryo is well formed 


.development comes to a standstill and the sporangium with the 
enclosed embryo is set free. This structure is the seed. The 
seed-bearing plants are called Spermatophytes and form a 
branch coordinate with Thallophytes, Bryophytes and Pteri- 
dophytes. In the Spermatophytes alternation of generation 
occurs as in the Archegoniates but the gametophyte is re- 
duced even further than in the higher Pteridophytes. 

293. Class i. Gymnospermae. In the Gymnosperms the 
sporangia are borne on the surface of modified leaves (sporo- 
phylls). The microsporophylls are arranged spirally on a 
short branch. The megasporophylls are usually similarly 
arranged. The microspore (pollen grain) develops a rudimen- 
tary prothallus of from one to three cells and the sperm cells 
reach the megaspore through a tube developed by the game- 
tophyte. The megaspore (embryo sac) develops a prothallus 
of many cells and several archegonia. The latter consists of 
a large egg cell and a few small neck cells. 

294. Order i. The Cycadince are tropical palm-like plants, with an 
tmbranched trunk and a rosette of pinnate leaves. The sperm cells bear a 
spiral band of cilia, The embryo consists of a suspensor, two cotyledons, 
a plumule and a hypocotyl. 

295. Order 2. The Ginkgoina contain only the Japanese genus Ginkgo, 
a deciduous tree with small fan-shaped palmately veined leaves. The 
sperm cells are ciliated. The embryo forms no suspensor. 

296. Order 3. The Coniferce comprise the "evergreens" and a few 
deciduous trees. The pines, cedars, spruces, hemlocks, cypresses and 
junipers are familiar examples. In the structure of the stem they differ 
from most Spermatophytes in the absence of tracheae. The tracheids are 
highly developed and take the place of tracheae. The sperm cells are not 
ciliated. The embryo forms a suspensor. 

297. Order 4. The Gnetince contain only three genera of exotic plants. 
They constitute in many respects a connecting link between the Gymno- 
sperms and Angiosperms. 

298. Class 2. Angiospermse. The Angiosperms include 
all the true flowering plants. The most distinctive character 


of the group is the pistil, a megasporophyll so formed as to 
entirely enclose the megasporangia. The flower consists of 
several circles of sporophylls surrounded by several circles of 
specially modified floral leaves. All these leaves are set close 
together on an extremely short axis. The typical flower bears 
at its apex or centre a circle of megasporophylls, around this 
two circles of microsporophylls and around these again two 
circles of floral leaves. Both circles of floral leaves may be 
colored or only the inner one. 

299. The male gametophyte is represented only by a single 
pollen tube nucleus and a sperm mother nucleus which divides 
into two sperm nuclei. No division into cells occurs. The 
female gametophyte is represented by the endosperm and the 
archegonium by the egg apparatus (two synergids and the egg 

300. The first seven orders of Angiosperms are Monocoty- 
ledonous (see page 71). The Dicotyledons contain thirty- 
three orders. 

301. For a detailed description of flowering plants the student 
should consult a manual of botany. 



I. Organization of Animals 

90. Protozoa: 

(a) Observe an actively moving amoeba for some time and sketch its 
outline five times to show the change of form. Trace in these 
outlines the changes through which each pseudopodium passes. 
Note the ingested food particles and, if possible, observe the 
process of ingestion. Note the contractile vacuole. 

(b) In a stained preparation note the structure of the protoplasm, 
the nucleus, the contractile vacuole and the food vacuoles. 

91. (a) Study the movement of a ciliate protozoan (Paramecium). How 

many kinds of movement does it perform. 

(b) Study a living individual under higher magnification. Note the 
cilia, the buccal groove leading to the mouth, the food vacuoles 
and the contractile vacuoles. 

(c) In a stained preparation note the macronucleus and the micro- 

92. Ccelenterata: 

(a) Observe a living hydra in the aquarium, first with the unaided 
eye then with the lens. 

(b) A hydra (living or a fixed preparation) which shows reproduction 
by budding. 

(c) Preparations of hydra to show the gonads. 

(d) A cross section of the body to show ectoderm and endoderm. 
Note the muscle fibrils which show as dots between ectoderm 
and entoderm. Also the central gastro-vascular cavity. 

93. (a) A hydroid colony (Obelia) (Pennaria). Sketch the colony. 

Compare a polyp with hydra. Is there evidence of budding? 
(b) A hydrozoan medusa (Obelia). In a stained preparation note 

the manubrium and mouth, the radial canals, the gonads, the 

tentacles and the velum. 
(b 7 ) The medusa of Gonionemus is larger than that of Obelia and may 

be studied with the lens. 


94. Annelida: 

(a) If living Nereis is at hand study its movements. 

(b) In a dorsal view of Nereis note general form of body; head; sen- 
sory, locomotor, and respiratory appendages; segmentation; 

(c) In a small living specimen the dorsal blood vessel may be seen. 
Note its rhythmical contractions. Note the direction of the 

(d) Study the head with a lens. Note the proboscis, tentacles, palps, 
cirri and eyes. 

(e) Study a segment cut from the middle of the body. Note the 
four large muscle masses, the intestine, the body cavity, the 
dorsal and ventral blood vessels, the ventral nerve cord and the 

(f) On a parapodium note its two divisions dorsal and ventral 
rami each bearing a cirrus, a ligula, setigerous lobes, setae and 
an acicuhim. 

(g) In a portion of the body from which the dorsal wall has been 
removed, note the intestine, the body cavity and the mesenteries. 

(h) In a microscopic preparation study the ova. 

95. Compare the earthworm (Lumbricus) with Nereis. 

96. Some of the smaller fresh-water annelids may be studied living, as 
transparent objects, under the microscope. 

97. Arthropoda: 

(a) If living crawfish (Cambarus) or lobsters (Homarus) are available 
study the movements. Note how the tail fin is used in loco- 
motion, also the legs. By adding a little carmine to the water 
can you detect any respiratory currents? What causes them? 
Feed with an earthworm and note activity of sense organs and 
the method of ingestion of food. 

(b) In a dorsal view note type of symmetry and character of seg- 
mentation of the body. 

(c) In a lateral view note in the cephalo thorax: rostrum, head, nuchal 
groove, thorax. In the abdomen: segments (No. ?), telson. The 
appendages of the cephalothorax are: i antennules, 2 antennae, 
(eyes), 3 mandibles, 4 first maxillae, 5 second maxillae, 6 first 
maxillipeds, 7 second maxillipeds, 8 third maxillipeds. (For 
appendages 3 to 8 see d). 9 Chelae, 10-13 ambulatory appen- 
dages. Are these all alike? The appendages of the abdomen 
are: 14-18 pleopods, 19 uropods. Are the pleopods all alike? 


Compare pleopods of male and female. Study with a lens a 
pleopod of segment 1 6 or 17; there is a basal protopod, a lateral 
exopod and a medial endopod. 

(d) Study the region of the mouth in a ventral view, to show espe- 
cially the appendages 3-8 (see c above). Begin with appendage 
8 and work forward. 

(e) In a ventral view of the entire animal note the openings of the 
green glands on the basal segments of the antennae, the openings 
of the gonoducts at the base of the eleventh (female) or thirteenth 
(male) appendages, and the anal opening. Draw the appendages 
9-13 in detail, showing all the joints. 

(f) Note the sensory hairs. Where are they found? In the eyes 
note the eye stalk and the retina. Study a preparation of the 
retina with the microscope. With the point of a needle search 
the dorsal surface of the basal segment of the antennule for the 
opening into the statocyst. 

(g) In a specimen in which the gill chamber has been laid open 
note character of the gills, their number, position and mode of 
attachment to the body. With a lens study a single gill under 
water in a watch glass. 

(h) In a specimen that has been sectioned longitudinally near the 
median plane note: (i) the digestive tract with oesophagus, 
cardiac and pyloric portions of the stomach, the liver and the 
intestine; (2) the heart in the pericardial chamber under the 
posterior edge of the thoracic shield and the arteries (ophthal- 
mic, sternal and abdominal), (3) the gonads lying below the 
heart, (4) the green gland above the basal joint of the antenna, 
(5) the large complicated muscles of the abdomen, (6) the ventral 
nerve cord communicating with the brain. 

(i) In a side view of a grasshopper (Schistocerca) show in the body: 
head, prothorax, mesothorax, metathorax, abdomen. The 
appendages are; antennas, lab rum, maxillae, with maxillary palps, 
labium with labial palps, legs. The parts of a leg are; coxa, 
femur, tibia and tarsus. 

(j) Remove the wings and draw the thorax and abdomen on a larger 
scale. Note especially the tympanum and the ten spiracles 
eight on the abdomen and two on the thorax. Note the number 
of segments in the abdomen and compare male and female. 

(k) Draw both wings of one side expanded. 


(1) In an anterior view of the head show the compound eyes, ocelli, 

antennae. Raise the labrum to expose the mandibles, 
(m) Compare Schistocerca with Cambarus. 
(n) Study a wasp as above i 1. 
98. Vertebra ta: 

(a) Draw a dorsal view of a fish (Perca) to show type of symmetry. 
Label as in b. 

(b) In a lateral view note: 

1. Head with mouth, nostrils, eyes (eyelids?), ears (?), operculum. 

2. Body with dorsal and anal fins (count the number of spines 
and soft rays) and paired pectoral and pelvic fins. The 
lateral line. 

3. Tail and tail fin. 

(c) Note the arrangement of the scales. Remove some and study 
with the lens. 

(d) Study the texture of the skin. 

(e) In a specimen from which the operculum has been removed 
study the gills. Note the gill arches and the gill rakers and 
the gills. Compare this respiratory system with that of the 

(f) -In a median longitudinal section of the lamprey (Petromyzon) 

note especially the notochord, also the brain and spinal cord, 
the mouth, pharynx and gill slits, oesophagus, stomach (?), 
intestine, liver, heart, kidney, gonad and gonoduct. 

(g) In a lateral view of the entire skeleton of any mammal show : 

1. Skull and mandible. 

2. Spinal column divided into cervical, dorsal, lumbar, sacral 
and caudal regions. Count the number of vertebrae in each 

3. Ribs (number?) and sternum. 

4. Girdles. 

A. The pectoral girdle consisting of a scapula and a clavicle 

on each side. 
P. The pelvic girdle consisting of ilium, pubis and ischium 

on each side. 

5. The appendages: 

A. Humerus, radius and ulna, carpals, metacarpals, phalanges 

(how many?) digits (how many?). 
P. Femur, tibia and fibula, tarsals, metatarsals, phalanges 

(how many?) digits (how many?). 


(h) Draw both ventral and lateral views of a skull. Show the sutures 

and identify the bones, 
(i) Draw lateral, dorsal and anterior views of a dorsal or lumbar 

vertebra to show centrum, neural arch, dorsal spine, transverse 

processes and articulating processes, 
(j) Draw a cross section of bone from a prepared slide. Note the 

Haversian canals, the lamellae, the lacunae and canaliculi. 
(k) In the leg of a frog from which the skin has been removed note 

how the fleshy mass is composed of distinct muscles. Note also 

the tendons and the relation of muscle to bone. Between the 

muscles may be found nerves and blood vessels. 
(1) Study cross striped muscle fibres in a prepared slide, 
(m) In a median longitudinal section of a dog fish (Galeus or 

Mustelus) study carefully the brain and spinal cord. Note also 

the vertebral column and compare the other organs of the 

body with those of lamprey as in f. 
(n) In another specimen which has been dissected to show the 

cranial nerves and brain identify the following: 

1. Brain: Olfactory lobes, cerebrum, optic lobes, cerebellum, 
medulla ollongata. 

2. Cranial nerves: I Olfactory, II Optic, III Oculomotor, IV 
Trochlearis (slender), V Trigeminal, VI Abducens (slender), 
VII Facial, VIII Auditory, IX Glossopharyngeal, X Vagus. 

3. Spinal cord and spinal nerves. 

(o) Draw dorsal and lateral views of the brain of a mammal showing 

cerebral hemispheres, cerebellum, and medulla, 
(p) Slit the skin of a frog along the mid-dorsal line, lift the skin of 

one side and note the median dorsal cutaneous nerves passing 

out to the skin, 
(q) With a lens search the inner surface of a piece of skin of a frog and 

observe the white nerves, the veins and the arteries, the three 

often running parallel, 
(r) Study the digestive system of a frog or turtle. Note : oesophagus, 

stomach, small intestine, liver, gall bladder, pancreas, large 

intestine. Note also the mesentery and the spleen, 
(s) Study as in r the digestive system of some mammal, 
(t) On the surface of a tongue (mammal) find the circumvallate and 

fungiform papillae. If present note also the character of the 

glottis and epiglottis. 


(u) Study the internal surfaces of a mammalian stomach and 

(v) The heart of a mammal. Sketch the organ as a whole showing 

auricles, ventricles and the aortic arch, 
(w) In a freshly killed turtle observe the beat of the heart noting the 

order of the beat in auricle's and ventricle, 
(x) Observe the circulation of the blood in the tail of a tadpole, the 

web of a frog's foot or the gills of a larval amphibian, 
(y) In an injected frog trace the principal arteries, viz: The truncus 

arteriosus which divides into three arches: 

1. The Carotid Arch with its branches. 

(a) The external carotid. 

(b) The internal carotid. 

2. The Systemic Arch with its branches. 

(a) The subclavian. 

(b) The dorsal aorta from which arise: 

1. The cceliaco-mesenteric. 

2. The urinogenital. 

3. The iliac. 

3. The Pulmo-cutaneous with its branches. 

(a) The pulmonary. 

(b) The cutaneous. 

(z) An injected mammal may be studied as in y. 

(a') Study the lungs of a frog or turtle from which the liver and 
stomach have been removed. Trace the trachea and bronchi 
from the glottis to the lungs. Study the internal structure of a 
lung which has been laid open. 

(b') In preparations of a mammalian lung note the structure of the 
trachea, the division of the lung into lobes and the internal 
structure of a lung. 

(c') In a male frog from which all other organs have been removed 
observe the testes, the kidneys and the ureters. 

(d') In a female frog observe the ovaries and the oviducts. 

(e') Compare a mammal (rat) with the frog with regard to the excre- 
tory and ^reproductive organs. 


99. The following outlines may be used to extend the laboratory 
studies to some of the other more important phyla and classes. 


100. Porifera (Sponges): 

(a) Study a simple sponge like Grantia. Note the general form, 
the point of attachment, the large excurrent opening or osculum 
and the spicules. 

(b) In a dry specimen of Grantia cut longitudinally, note the central 
cavity or cloaca and the radial canals. Note also the form and 
arrangement of all spicules. 

(c) A longitudinal section treated with acid to remove the spicules 
and then stained and mounted will show the relation of the 
radial canals and the incurrent canals or interradial spaces. 

(d) A dry specimen in cross section should be studied in connection 
with b. 

(e) A cross section with the spicules, stained and mounted, will 
show further details especially with regard to the arrangement 
of spicules and may also show ova or embryos. 

(f) The spicules may be set free by dissolving the fleshy parts in 
boiling potash. How many kinds of spicules are there? 

101. Cnidaria: 

(a) A sea anemone (Metridium). In a lateral view note: the col- 
umn, the base, the crown of tentacles. 

(b) In an oral view note the mouth with the grooved lips and 
siphonoglyph (one or two). 

(c) In a cross section through the middle of the column note: 

1. The gullet with grooves and siphonoglyphes. 

2. The gastro-vascular cavity incompletely divided into cham- 
bers by the mesenteries. 

3. The mesenteries are of two kinds, complete and incomplete 
and on their edges may be found the mesenterial filaments 
and acontia and the gonads. 

(d) Study a fragment of a coral (Astrangia). The cups (theca) each 
contain a central columella and a number (?) of radial septa. 

1 02. Platyhelminthes: 

(a) In a living planarian note the method of locomotion, the eye 
spots, the proboscis. 

(b) In an adult liver fluke note the terminal mouth and the ventral 

(c) In a tape worm note: 

1. The scolex with a circlet of hooks and suckers. 

2. The body of proglottides. Note the form of a proglottis in 
different regions of the body. 


(d) A proglottis cleared and mounted may show the reproductive 
organs, viz: ovary, shell gland, vitelline glands, uterus, testes 
and genital pore. 

103. Aschelminthes : 

(a) Note the form and movements of a living "vinegar eel" or a 
thread worm from an aquarium. 

(b) In Ascaris note the form of the body, the terminal mouth with 
its lips, and the anus. If the body is slit open the intestine and 
reproductive organs may be identified. 

104. Annelida: Study a living leech in water. Note its method of swim- 
ming, and locomotion by means of its suckers. 

105. Echinodermata: 

(a) In an aboral view of a starfish (Asterias) note the type of 
symmetry, the central disc and the arms or rays. On the 
general surface will be found the hard spines and soft papulae; 
on the disc the madreporic plate. 

(b) If a living specimen can be had study it in sea water for the 
method of progression. 

(c) On the oral surface are the mouth, the ambulacral grooves 
with the ambulacral feet, the radial nerve ridge in the middle 
of each ray and, at the tip of each arm, a tentacle and eye spot. 

(d) If the aboral wall is removed the stomach and hepatic caeca 
come into view and beneath these the gonads and the ampullae 
of the ambulacral system. Note also the structure of the skel- 
etal system. 

(e) Compare a sea-urchin with a starfish. 

(f) Compare a sea-cucumber with a starfish. 

106. Arthropoda: 

(a) Compare a crab with the crawfish. 

(b) Observe some living fresh water Entomostraca with the micro- 

(c) Study a "thousand-leg" or centipede. 

(d) Study a spider. 

(e) For further studies on insects consult a work on entomology. 

107. Mollusca: 

(a) Study the method of locomotion of a common snail. Note 
symmetry of body and shell. 

(b) In a right lateral view note the head, the foot, the collar 
and shell. Note also the mouth, the tentacles, the eyes, the 
respiratory opening. 


(c) In a small living snail the movement of the heart may be seen 
through the shell. 

(d) When the shell is removed the lung chamber may be laid open. 
Note also the heart, kidney, liver, coils of the intestine and, at 
the top of the spiral mass, the gonad. 

(e) If the dorsal body wall is removed from the tentacle to the heart 
the following organs come to view: The buccal mass, the ces- 
phagus surrounded by the nerve collar, the crop with the sali- 
vary glands at either side, the complicated reproductive organs 
lying on the right side of the body. 

(f) Draw a dorsal view of a clam. Note the symmetry. (The 
hinge is dorsal and the siphon posterior.) 

(g) The clam may be studied with one valve of the shell removed. 
Note: The mantle, the two adductor muscles and the siphon 
with its two openings. At the dorsal edge of the mantle is the 
pericardial cavity in which lies the heart. 

(h) Raise the mantle and observe the large chamber in which lie 
the visceral mass and the fleshy foot. Upon the visceral mass 
lie the two gills and at the anterior edge the palps which hide 
the mouth. 

(i) Draw a lateral view of a squid. Note the head with the arms 
and eyes. Behind the head are the collar and funnel. The 
body is covered with a very thick mantle which is expanded 
at the end into a fin. 

(j) Study the suckers on the arms. Find the mouth and jaws. 

(k) The visceral mass and the gills are exposed by slitting open the 
mantle on the ventral side. 

(1) The rudimentary shell is embedded in the dorsal surface of the 


302. Animals and Plants. Animals present a much greater 
variety of types of organization than plants. This is largely 
because the higher animals are vastly more complex than the 
higher plants. The apparent complexity of a tree, for example, 
is in reality due chiefly to a repetition of similar parts; but in 
animals there is a progressive differentiation of parts from the 
lowest to the highest forms, so that even a single cell may have 


a structure and function not duplicated by any other cell in the 
body. The gap between the simplest and most complex 
animals is occupied by many types of intermediate degrees of 
complexity and we shall therefore keep in mind several of the 
most significant of these types while seeking to obtain a concep- 
tion of what constitutes an animal. 

303. Animal Types. As an example of the very simplest 
kinds of animals we shall frequently refer to the amoeba, a 
minute speck of living jelly, quite common in the bottom slime 
of ponds. 

304. As a slightly more complex form we will take hydra, 
which is also found in fresh-water ponds, attached to plants 
or other objects in the water. It is vase-like in form and has a 
circle of long slender arms or tentacles near the oral end. 

305. As a still more complex form the common earth- 
worm may serve very well, or a segmented marine worm, like 

306. The crayfish will form another step forward. This 
animal is also common in most parts of the world and should 
be familiar to everyone. In this connection reference will 
occasionally be made to insects, which belong to the same 

307. As an example of the most complicated type of 
animal organization any mammal may be kept in mind, such 
as the cat, dog or rabbit, or, better still, man. The student 
will be supposed to have some knowledge of human anatomy 
and physiology. Reference will also be made to other members 
of the vertebrate phylum, such as fishes, frogs, reptiles and birds. 

308. Color and Form. If we compare plants and animals 
with regard to color and form we find nothing in common. 
Animals contain no chlorophyl and they are therefore physio- 
logically dependent upon plants. Nor do they have any other 
general color characteristic. The form of plants we found was 
determined by the necessity of exposing chlorophyll tissue to 


the light, and for maximum efficiency this demands a branching, 
or what may be called a diffuse form of body. Since animals 
do not demand such light exposure the branching form of body 
is also not necessary. As a matter of fact the animal body is 
not only not diffuse, it is constructed in the most compact 
manner possible. The reason for this is, of course, not far to 
seek. It is demanded by the most distinctively animal char- 
acteristic locomotion. For the purposes of respiration animals 
also require a large exposure of surface to the surrounding 
medium, but this is secured in the gills and lungs by folding 
surfaces in such a way as to make the respiratory organs occupy 
very little space in proportion to the surface which they expose. 
This of course secures protection to the organs but at the same 
time it also allows greater freedom of motion. That the latter 
is an important consideration is evidenced by the fact that many 
fixed animals are also diffuse in form. 

309. Locomotion. The ability to move from place to place 
is the most conspicuous animal character, but coordinate with 
it and inseparably connected with it is sensibility to external 
influences. This latter character is not wanting in plants but 
it is so much more greatly developed in animals as to amount 
practically to a different thing. Locomotion and sensibility 
go hand in hand because locomotion without sensibility would 
be aimless and sensibility without the power of motion would 
be without value. In this connection " motion" or " loco- 
motion" must be understood in a broad sense as a muscular 
response which may involve only a part of the body. The power 
of locomotion carries with it a large train of interesting conse- 
quences which determine the form and structure of the animal 
even to the minutest detail. These will be considered at 
various points as the subject develops, but here we will examine 
only into the matter of the external form as resulting from 

310. Axis of Locomotion. In the more primitive condition, 


differentiation results in a repetition of similar parts and these 
parts must either be arranged radially or serially. But the 
serial arrangement results in an elongated body and this is 
better adapted for free locomotion. Consequently the body 
of the typical animal is elongated in the axis of locomotion. 

311. Cephalization. Since locomotion is generally in a 
horizontal direction the elongation of the body is horizontal. 
But the two poles of this body are not alike, because the prin- 
ciple of division of labor and efficiency would make locomotion 
in one of the two directions become the principal direction of 
locomotion. The animal usually moves with the same end 
forward and this end, which is called anterior, is very different 
from the opposite or posterior end. The difference is chiefly 
due to the development of special sense organs at the anterior 
end, because this end comes more positively into relation with 
the forces which affect the senses. The development of the spe- 
cial sense organs further carries with it the special development 
of the central nervous system of that region; that is, the devel- 
opment of a brain. The locomotion of the animal has to do 
largely with obtaining food and this probably determines that 
the anterior end is located near the mouth. Then the develop- 
ment of organs for ingestion and comminuting food, and the 
sense organs connected with this function still further differen- 
tiate the anterior end from the posterior. The development of 
all these organs at the anterior end of the animal forming a com- 
plex of organs called the head is called cephalization. The 
posterior end of the body is sometimes developed into an organ 
of propulsion or otherwise specialized, but never to the degree to 
which the more positive conditions bring the development of 
the anterior end. In this way are determined the elongation 
of the animal with its principal axis horizontal, and the differ- 
entiation of the two poles into anterior and posterior. 

312. Dorsal and Ventral. For animals which pass from one 
medium to another, as from water to dry land or from the latter 


into the air, two sets of locomotor organs might be necessary, 
but in general the principle of economy determines that only 
one set of appendages is developed. For those forms which 
move on the bottom of the sea or on land the locomotor ap- 

FIG. 62. Diagram of bilateral symmetry (fish), d-v, Dorso- ventral axis; 
r-l, right-left axis; ap, appendages; b.c., body cavity; ch., notochord; d.f., dorsal 
fin; g, intestine; h, heart; h.a., haemal arch; m, muscles; n.a., neural arch; sp, 
spinal cord; v.c., vertebral column. (From Galloway.) 

pendages will necessarily be constructed with reference to the 
force of gravity. Since this force operates in one direction 
only, the appendages have a one-sided relation to the body. 
There are therefore an upper and a lower side of the body, and 
these two sides are not alike. Moreover, since light, which 


also affects the organism, impinges more strongly from above, 
it will also operate to differentiate the upper and lower sides. 
These two sides are distinguished as dorsal and ventral, 

313. Fishes which do not rest on the bottom but always 
float suspended in the water do not present the same degree 
of dorso-ventral differentiation. In this case the action of 
gravity is practically eliminated by the buoyancy of the water. 

314. Right and Left. With the differentiation of anterior 
and posterior and of dorsal and ventral the animal comes to 
have a right side and a left side. These two sides are so related 
to the external world that every force which acts on one side 
affects the other side also in a symmetrical way. In con- 
sequence, the two sides are also symmetrical in form in every 

315. Bilateral Symmetry. An organism like the one just 
described is divided into right and left symmetrical halves by 
the vertical plane in which the principal axis lies. No other 
symmetrical division of such a form is possible. A body 
having such a form is said to be bilaterally symmetrical, and 
this is the type of symmetry found in most animals and gener- 
ally in those having marked freedom of locomotion. 

316. Radial Symmetry. Those animals which are very slug- 
gish in movement or actually fixed, show little or no evidence oi 
bilateral symmetry. The principal axis is perpendicular to the 
substratum, and its two poles are differentiated; the mouth and 
associated organs for ingestion are at the free, oral, pole, while 
the opposite, aboral, pole is modified for attachment. If the 
animal is not actually fixed, the oral pole may be toward the 
substratum. In either case, the organization of the animal is 
more or less perfectly radial with respect to the principal axis, 
the number of rays being 2, 4, 6 or 5 or a multiple of one oi 
these numbers. The oral-aboral differentiation in part cor- 
responds to the dorso-ventral differentiation of bilateral forms. 



The radial symmetry is to be referred to the radial action of the 
environment, which is the same in all directions at right angles 
to the principal axis. This type of symmetry is characteristic 
of plants, and inasmuch as these radial animals approach plants 
in their life habit, they are affected by their environment like 
plants and consequently approach plants in their structure. 
A b 

B ab. o. 

FIG. 63. Diagram of radial symmetry as represented by a medusa, 
view; B, lateral view; o-ab.o., principal axis. 


A, Oral 

317. Universal Symmetry. In case external forces are the 
same in all directions the corresponding response form would be 
a sphere. This might be called universal symmetry. Such a 
condition is actually approached only by a few protozoa which 
float in the water, unattached, and are continually turning over 
and over. 

318. Asymmetry. Varying degrees of asymmetry are found 
among fixed or sluggish types. This is more often true of 
colonial forms which grow by a process of budding and become 
asymmetrical by unequal growth. In these cases, however, the 
individual may be perfectly radial. 

319. A study of some exceptional cases will serve to "prove 
the rule." The shell of gasteropod molluscs is asymmetrical. 
However, when the animal is completely withdrawn into the 
shell and is then completely asymmetrical, it is also to all in- 


tents, so far as external forces are concerned, an inert body and 
has lost its animal character completely. When expanded and 
moving, on the other hand, the animal character reappears and 
at the same time the form of the animal becomes largely or 
completely bilaterally symmetrical. 

320. The free swimming larvae of the Echinoderms are per- 
fectly bilateral, but when they assume the less active or fixed 
life habit of the adult, they become radial in symmetry. This 
change involves a radical metamorphosis. In the case of some 

FIG. 64. The bilaterally symmetrical free swimming larva of an Echinoderr 
(From Ziegler's models.) 

of the Holothuria a second change occurs, in which the radii 
symmetry is largely superseded by a secondary bilateral sym- 
metry. This is brought about by the habit of the animal 
assuming a horizontal instead of a vertical position of th< 
principal axis. 

321. The adult ascidians and barnacles also show a stroi 
tendency toward radial symmetry, although the active lam 
are bilateral. 

322. A striking example of a different type is offered by th< 
"flat fishes," such as the flounder and sole. The young 
these fishes have the ordinary type of bilateral symmetry, am 
in swimming they also assume the erect position characteristi< 


of fishes. But they soon turn on one side and in the adult 
continue in this attitude, with one side toward the earth. In 
this case the principal axis is maintained in the same relative 
position, but the dorso-ventral and right-left axes are trans- 
posed in space. In response, the form of the animal also under- 

FIG. 65. A sea-urchin (Clypeaster) in which a secondary bilateral symmetry 
is impressed on a radial organism. Oral view, slightly reduced. 

goes a change, so that the dorsal and ventral surfaces become 
symmetrical and the right and left sides unsymmetrical. The 
plane of symmetry is thus revolved 90 on the principal axis. 
It would be more correct, however, to say that while the animal 


revolves 90 on its principal axis, its plane of symmetry remains 
fixed. That is, of course, what one should expect following the 
general principles already laid down; for since there is no change 
in the external forces which cause symmetry in the organism 
there should be no change in the plane of symmetry following 
the revolution of the animal on its axis. 

FIG. 66. The flounder, Pseudopleuronectes Americanus, showing approximate 
dorso-ventral symmetry. Note that both eyes are on the right side of the head. 
(From Hegner, after Goode.) 

323. Size and Differentiation. Animals vary greatly in size 
and complexity of structure, from the microscopic protozoa to 
the gigantic mammals, and it is of interest to note that, in a 
general way, size and complexity vary together. Superiority 
in size is of itself an advantage, especially where there is a 
contest between individuals. But more important is the advan- 
tage derived from complexity, which permits of differentiation, 
division of labor and consequent efficiency. Considerable 
differentiation may be found between the parts of the same cell, 
as in the protozoa, but when the body is composed of many 
cells the differentiation may be vastly greater, both in regard to 
the number of kinds of differentiation and the degree to which 
it is carried. Thus the functions of contraction, irritability, 


digestion, etc., may be taken up by the different cells or groups 
of cells and performed more efficiently. 

The various kinds of differentiation ' may be grouped under 
the following heads : 

324. Integument. An extremely important set of organs 
are the various protective structures which cover the entire 
surface of the body of all but the very lowest animals. The 
most important of these structures are the hair, feathers, scales, 
bony plates, cuticular secretions, shells, glands and the un- 
modified skin itself. These together comprise the integument. 

325. The Nerve-Muscle Mechanism. The nervous and the 
muscular tissues are the most highly differentiated tissues of 
the body. For efficiency the sense organs must be very numer- 
ous, so that on the surface of the body there is scarcely a point 
large enough to be visible which is not occupied. The muscles 
for strength must be massive. The central nervous system, 
through which all the various organs of the body are brought 
into harmonious relations, especially the sense organs and the 
organs of response, is the most complicated organ of the body 
and is also of considerable size. The weight of a large body 
requires special supporting structures, and for the most efficient 
application of muscular energy for locomotion, a system of 
levers is necessary. These structures comprise the skeleton 
and the connective tissues, which together constitute the largest 
set of organs in the body. 

326. Digestion. The highly differentiated tissues just de- 
scribed have surrendered the function of digestion to other cells 
of the body, and these are connected with the central cavity, 
in which digestion takes place. The various phases of the 
digestive process are separated and distributed to distinct 
groups of cells, composing as many organs. These together 
constitute the digestive system. 

327. Circulation. The digested food is absorbed by the 
digestive tract in much larger quantities than is necessary to 



nourish the tissues of the digestive tract itself. It is in a fluid 
state as blood plasma and is available for assimilation by the 
other tissues of the body. But a large part of these tissues is 
too far removed from the digestive tract to be nourished by 
transfusion. A system of vessels becomes necessary for the 
conduction of the blood plasma and, in addition, a heart to 
force the blood along. 

328. Respiration. The smaller organisms absorb enough 
oxygen through the general surface of the body to supply the 
needs of all the tissues. But increase in size also brings some 
of the tissues too far from the surface to be supplied in this way. 
Moreover, the development of impervious integumentary 
tissues prevents the absorption of much oxygen through the 
general surface. There then becomes necessary a special 
respiratory organ gills or lungs. From the organs of 
respiration the oxygen can reach the tissues through the 
channels followed by the blood plasma, either dissolved in 
the blood plasma or carried by special vehicles, the red blood 

329. Excretion. The waste products of metabolism are 
voided by the smallest animals through the general body sur- 
face. But this also becomes impossible in the higher animals, 
where they are taken up by the blood and are then in part elimi- 
nated through special excretory organs, the nephridia, kidneys, 

330. Reproduction. Most of the protozoa reproduce by 
division of the body, or by budding. These methods also 
occur largely among the lower metazoa, but highly differentiated 
tissues lose the power of division and the more complex animals 
have not the power of reproducing in this way. In them there 
is a special organ, in which undifferentiated tissue is reserved 
for reproduction. 

331. Organization of the Body. We thus see that the de- 
velopment of the animal, i. e., the nerve-muscle mechanism, 


to the highest degree involves the development of an organism 
made up of nine systems of organs: 










332. "Higher" and "Lower" Animals. The functions per- 
formed by these nine systems of organs are all performed by 
the undifferentiated protoplasm of the amoeba and must like- 
wise be provided for by every other animal. The higher forms 
are, therefore, not distinguished by the development of new 
functions, but by the effectiveness with which these functions 
are performed. Under favorable conditions the functions 
of the amoeba are equal to the demands made upon them, but 
with a serious change in these conditions they fail and the amoeba 
comes to naught. Against drouth, heat, cold, the lack of food 
in the immediate vicinity and the attacks of larger animals 
the amoeba has no defense. On the other hand, a higher animal, 
say, e. g., a wolf, is effectually protected against dessication by 
his skin. Ordinary climatic changes of temperature are auto- 
matically compensated for and the body maintains an equable 
internal temperature. When food fails he travels far in search 
of more, and when attacked he knows how to defend himself. 
Indeed, he is a living demonstration of his superiority, for his 
life is maintained by the destruction of other living things 
which are not able to defend themselves against him. He 
demonstrates his superiority, and we habitually distinguish 
such capable forms from the less capable by the terms higher 
and lower. But there is still another way in which the wolf 



demonstrates his superiority. The life of the amoeba is brief 
in time as well as circumscribed in space; the range of its experi- 
ences are as limited as its sensibilities are vague and the memory 
of them instantly vanishes. The wolf, however, lives on for 

days, months and years. His 
highly specialized nerve cells not 
only feel infinitely more acutely 
but they are able to retain impres- 
sions, and through his compara- 
tively long life these accumulated 
experiences are made to serve to 
his advantage. The intelligence 
of the wolf goes far to make him 
independent of his environment 
and he thereby demonstrates his 
superiority. The highest animals 
are those most completely inde- 
pendent of the conditions under 
which they live. 

333 . Segmentation. Compar- 
ing the larger and smaller animals 
again from another point of view: 
The greater size of the body may 
be due to larger organs, or it may 
also result from a repetition of 
similar organs, as in plants. The 
former condition is well exempli- 
fied by the phylum Mollusca, in 
which the repetition of similar 

organs (in this sense) does not occur, and yet one class of 
this phylum (Cephalopods) has attained a high degree of 
development, and counts, in some of its species, animals of the 
greatest size. 

334. When repetition of organs occurs it may affect some 

FIG. 67. The anterior end of 
an annelid, Nereis. The body 
consists of upward of 130 similar 
segments or metameres. Hence 
the animal is said to be homon- 
omously segmented. 


systems of organs more than others, but usually there is a 
tendency for the various systems to be repeated in the same 
degree. In this way the body is divided into segments, each one 
of which contains a segment of each system of organs. In 
elongated animals the body segments are arranged in a linear 
series, and are called metameres. Within the metamere each 
system of organs is represented by a single segment if the system 
is median in position, or by a symmetrical pair if they are lateral. 

335. The segments of radial animals are called antimeres 
and they are arranged radially about the principal axis of 
symmetry. The antimeres are also bilaterally symmetrical. 

336. Metameric segmentation introduces a new type of 
differentiation, the differentiation of segments. In the phylum 
Vermes there is little differentiation of segments and hence the 
segments are said to be homonymous. When the segments are 
differentiated they are heteronymous. This occurs in progres- 
sive stages through the phyla Arthropoda and Vertebrata, so 
that in the higher forms the segmentation is considerably 
obscured. Of course, differentiation of segments greatly in- 
creases the complexity of organization. 

337. Segmentation of the body is a means by which its 
flexibility may be provided for. This is of special importance 
in animals having a skeleton. 


338. The amoeba is said to be a naked cell, i. e., it has no cell 
wall, and therefore can scarcely be said to have an integument. 
The surface layer of the protoplasm, the pellicle, is slightly 
denser than that lying deeper, and its consistency is such as 
to maintain a well-defined boundary between the organism and 
the surrounding water. The protoplasm is so nearly the density 
of water and the animal so minute that little force is required 



to maintain the integrity of the body. The delicate pellicle 
is therefore sufficient for the amoeba, although for larger forms 
it would be entirely inadequate. 

339. The body wall of hydra consists of two layers of cells, 
an outer ectoderm and an inner entoderm. The cells of the 

FIG. 68. Amoeba Proteus. Na, A cluster of algal cells which is being engulfed 
by the protoplasm of the amceba; Cr, contractile vacuole; N, nucleus. (From 
Marshall, after Doflein). 

ectoderm are practically naked protoplasm on the exposed 
surfaces, but this protoplasm is so dense that it serves as an 
integument. In the closely related hydroids the ectoderm 
secretes a thin membrane known as the perisarc or cuticula. 
It is extremely tough and well adapted for protection and 

340. The epidermis of worms corresponds to the ectoderm 
of hydra, but is much thicker because of the columnar form of 



the cells composing it. It secretes a very thick and firm cuticula. 
Both epidermis and cuticula vary greatly in thickness in differ- 
ent species of worms, but this variation corresponds approxi- 
mately with the size of the worm. 

/* B 

FIG. 69. A, Diagram of Hydra; B, portion of the wall highly magnified; 
6, bud; ect., ectoderm; ent, entoderm; /, foot; //, flagellum; g.v., gastro-vascular 
cavity; m, mouth; mes, supporting lamella; m.f., muscle fibre of the ectoderm 
cells; n, nettling cells; n', same, exploded; ., nucleus; t, tentacle; v, vacuole. 
(From Galloway.) 

341. The integument of the crayfish is similar to that of the 
worm. The chief difference lies in the much greater thickness of 
the cuticula, which here consists of a peculiar substance called 
chitin. Chi tin is an extremely firm and elastic substance, 



FIG. 70. Diagrams of the integument. A, A sectional view of the epidermis 
on the superior ligula of nereis; B, a surface view of the same showing the cell. 


which is highly resistant to most chemical reagents. The cu- 
ticulaof the crayfish also contains considerable quantities of lime 
salts, which add much to its hardness. The integument of 
insects is similar to that of the crayfish, but the cuticula lacks 
the lime. 

342. This type of integument is extremely effective as a 
protective structure, and it will be noted that within the group 
of Arthropods, we first find animals which can withstand 
exposure to dry air. A disadvantage of the arthropod type of 
integument is that it is too rigid to permit great freedom of 
motion. For this reason the integument of both body and 
appendages is divided into segments which are connected by 
zones of more flexible tissue. Flexure of the body and appen- 
dages can only occur at these points. (See'ecdysis, page 346.) 

343. The skin of vertebrates, especially that of Mammals, 
is much more complex. The epidermis consists of many layers 
of cells, which are continually increasing in number by the active 
growth and division of the deeper lying ones. The superficial 
layers are composed of lifeless cells which have been transformed 
into flattened, horny scales. This horny layer takes the place 
of a cuticula. In addition to the epidermis there is a deeper 
layer, the dermis, which is much thicker and consists chiefly 
of felted connective tissue fibres. 

outlines; C, a sectional view of the thick epidermis of the ventral side of nereis, 
with the underlying circular and longitudinal muscles. In A the cells are 
cubical and the cuticula very thin; in C the cells are columnar, the cuticula is 
very thick and pierced by pores through which glands open and sensory hairs 
project. The lumina of the gland cells are represented as light oval spaces. 
D, a sectional view of the epidermis of the crayfish; E, the epidermis of a mammal 
with a hair follicle in which a hair (h) is beginning to develop; F, the epidermis 
of a mammal highly magnified and showing its layers horny layer, granular 
layer and Malpighian layer; G, a section through the skin of a mammal including 
epidermis, dermis, and a hair follicle; H, diagram of the epidermis of a reptile 
with its outer layers (black) solidified into horny scles; /, a similar diagram of 
the bony scales beneath the epidermis of a fish; /, a section of the skin of a 
bird to show a feather in an early stage of development. Bl.V., blood-vessels; 
C.M., circular muscles; Cu., cuticula; D, dermis; Ep, epidermis; h, hair; H.L., 
horny layer; l.m., longitudinal muscles; M, muscles; M.L., Malpighian layer; 
s (in G), sebaceous glands; s (in H and 7), scales; S.C., sensory cell. 


344. This type of integument is extremely flexible and there- 
fore does, not impede locomotion or other movements. At 
the same time it is very tough, fairly resistent to mechanical 
injury, and the lifeless superficial layers of the epidermis are 
impervious to water and thus protect the living parts from the 
dry air. 

345. In addition to these undifferentiated portions of the 
various types of integument there are also certain important 
specialized structures developed which serve as supplementary 
protective organs, as organs of defense and offense, as prehensile 
organs and as accessory organs of locomotion. 

346. The most common type of differentiation consists simply 
of a local thickening of the cuticula or epidermis. Thus in 
many worms minute tubercles or larger jaw-like structures are 
found on the walls of the mouth and pharynx. The setae and 
aciculae on the parapodia and sometimes scales on the back, are 
of similar origin. In insects and Crustacea the sensory hairs 
found especially on the antennae and mouth parts, and around 
the joints of the appendages and body are also produced by the 
unusually active secretion of chitin by one- or a few cells of the 
underlying epidermis. 

347. In Vertebrates the epidermis becomes modified in a 
great variety of ways by the aggregation of the minute horny 
scales into exceedingly firm structures, which serve a great vari- 
ety of purposes. Among the most important of these structures 
are nails, claws, hoofs, spurs, horns, beaks, " tortoise shell," 
" whale-bone," scales of certain kinds, hairs and feathers. The 
scales found on reptiles, birds and some fishes are merely thick 
and compact areas of the corneous layer of the epidermis. The 
hair differs from the scale only in its form. The feather may be 
likened to a hair greatly enlarged in diameter and hollow, with 
certain parts of its shaft splitting in a complicated fashion and 
thereby producing the vane. 

348. The scales of most fishes are not horn but thin plates 



of bone which are formed, not in the epidermis, but in the 
dermis. In some fishes there is deposited on the upper surface 
of the bony scale a layer of enamel which is due to the activity 
of the cells of the overlying epidermis. Teeth are also formed 
in this way, the dentine being merely a kind of bone. Antlers 
are bone and are formed by the dermis. 

349. Glands. Glands form another type of differentiated 
integumentary structures. They are of the simplest form in 
hydra and worms where they consist of single cells of the 
epidermis. These cells do not secrete a cuticula on their free 



FIG. 71. Glands. A, The ectoderm of hydra showing granules of cement (g) 
secreted by the cells at the surface; B, a section through the epidermis of nereis, 
showing a number of unicellular glands. Only the outlines of the outer ends of 
the gland cells are shown. The nuclear portions are below but not distinguish- 
able in the figure. Cu, Cuticula; Ep, epidermis; g, pores of the glands; C, 
diagram of a unicellular gland at the beginning of secretion; D, the same when 
swollen with secretion; E, the same cell after its contents are ejected. 

surfaces, but do secrete other substances which at first accumu- 
late within the bodies of the cells but are later forced out 
through the pores in the cuticula which were formed by the non- 
secretion of cuticula by the gland cells. The secretions accumu- 
late until the excess gradually oozes out through the pores or is 
suddenly forced out in larger quantities by the contraction of the 
surrounding tissues. In the case of worms and many other 
aquatic animals the substance secreted takes up water and forms 
slime. The function of the slime is probably in most cases 


protective, but in special cases it may serve a variety of other 
functions. Sometimes it serves to cement together the parti- 
cles of earth to form a tube in which the animal lives. Some- 

FIG. 72. A parchment-like tube constructed by a marine annelid, Chsetop- 
teris. The tube is buried in the mud, except an inch or two of each end which 
project above the surface. Both ends are open to permit a current of water to 
pass through. X 2/3. 

times it is used in locomotion, as in case of snails. The skin 
of fishes is richly supplied with slime glands. Hydra and 
many other animals attach themselves temporarily or per- 



manently by a secretion which acts like a cement. Leathery 
tubes are formed by many worms by a secretion which hardens 

FIG. 73. Oven-shaped shelters of the caddice-worm, composed of pebbles 
fastened together with silk fibres spun from the mouth. 

FIG. 74. Another type of shelter tubes of caddice-worms, composed of sticks 
and pebbles. The caddice-worm is the aquatic larva of the caddice-flv 

in the water into an exceedingly tough fibre. There is often 
considerable lime mingled with these secretions and sometimes 


the lime is deposited in great quantities. This is notably true 
of the corals, which are closely related to hydra. Many 

FIG. 75. A common type of massive coral, Solenastrea, from Bermuda. Each 
cup is formed by a polyp and radial septa in the cups indicate the pairs of 
mesenteries of the polyp. Slightly reduced. 

worms live in hard, limey tubes which they secrete. The shell 
of the mollusc originates in the same way, but here it forms part 
of the body of the animal. 


350. The epidermis of Crustacea and Insects is generally 
devoid of glands. Fishes and Amphibia are well supplied with 
slime glands. Reptiles have practically no glands in the skin. 
In Birds there is one important gland or group of glands. This 
is the uropygal gland, located on the tail. It secretes an oil 
which is transferred by the bird by means of its beak to the 
surface of the feathers when preening. The oil keeps the 
feathers flexible and prevents wetting. For this reason 
" water rolls off a duck's back/' as it also does from other 

351. The skin of Mammals is provided with two kinds of 
glands, sebaceous and sweat glands. The former are grouped 
around the hair follicles and their oily secretion escapes at the 
base of the hair. It not only serves to keep the hair flexible 
but also the corneous layer of the epidermis. This is a very 
important function, since the dead tissue would otb : ~e 
break and expose the living tissue beneath, which at 
happens in the chapping of an abnormally "dry" skiii v*ie 
oil also renders the skin impervious to water. 

352. The sweat glands are the thermostatic organs of the 
body. They will be discussed elsewhere. (Page 411.) 


353. Aru'mals which live in the water may sense their food 
in one or more of three ways, viz. : Sight, smell, and taste. For 
amoeba the first is excluded, since amoeba cannot be said to 
have a sense of sight. The sensations of taste and smell are 
both due to chemical stimuli; that is to say, the substances 
which stimulate the sense organs must be in solution, and the 
stimulation itself is a chemical process. The distinction be- 
tween the two sets of senses is largely a matter of position of the 
sense organs. The organs of taste are located in the mouth, while 
those of smell are elsewhere on the surface of the body, usually 



in the vicinity of the mouth. For amoeba no such distinction 
is possible, and we can, therefore, only speak of a chemical 
sense. Amoeba has been observed to engulf particles of sand 
and other inorganic substances which could not serve as food, 
but in spite of this fact there is much evidence to show that there 
is a chemical sense. Ordinarily the animal distinguishes food 
particles from others and has even been observed to follow a 
moving protozoan, which it also captured and devoured. Many 
observations lead to the general conclusion that the protozoa 
generally are sensitive to the chemical condition of the sur- 
rounding medium. They are attracted to or repelled from the 
source whence such substances are diffusing through the water. 
Strong stimuli produce decided responses, such as a contraction 
of the amoeba into a spherical mass. 

354. Amoeba is also sensitive to mechanical stimuli, such as a 
touch or a jar. By this means it "feels" the presence of a 
foreign object to which it may adhere. If the stimulus is 
irritating the response may take the form of a secretion of 
slime, the withdrawal of the pseudopodium or the complete 
contraction of the amoeba into a spherical mass. 

355. Amoeba is also sensitive to strong light though not as 
much so as some other colorless protozoa. Usually, when 
other things are equal, protozoa may be observed to seek a 
point where the light is neither excessively strong nor weak. 

356. To changes in temperature amoeba is also sensitive. 
With increase in temperature it becomes more active, until at 
about 35 C. it contracts and remains motionless. With a 
falling temperature activity is also lowered until at a little 
above o C. it ceases entirely, often without contraction. 
When possible amoeba will move out of a region of extremely 
high or low temperature to one more nearly normal. 

357. Though amoeba is sensitive to these various stimuli, 
yet it has no sense organs. There is no differentiation of organs 
for the reception of the different stimuli, nor yet for the re- 


ception of stimuli in general. Sensibility is one of the primary 
functions of undifferentiated protoplasm. 

358. Almost the same may be said of hydra. The organism 
is sensitive to the same stimuli and to no others. So far as is 
known there is no localization of sensory function. All the 
cells of the body consist largely of undifferentiated protoplasm, 
which is probably the organ of general sense as in amoeba. It 
is true that some of the cells of the ectoderm project beyond the 
surface by slender protoplasmic processes which are probably 
organs for the reception of stimuli. Since these processes are 
more exposed they are more readily affected by stimuli and 
hence may be regarded as incipient sense organs. 

359. In some other Coelenterates as, e. g., the medusae, the 
sensory cells and the nervous elements generally, are better 
developed. Many cells of the ectoderm are provided at their 
free surfaces with sensory hairs while the opposite ends of the 
cells are prolonged into long fibres which extend for some 
distance under the ectoderm. 

360. In nereis the sense cells are clearly differentiated 
from the other cells of the epidermis. Each sense cell projects 
through the cuticula by a single protoplasmic process, while 
the remainder of the cell is elongated into a fibre which extends 
deep into the body to the central nervous system. The nucleus 
of the cell often lies in the epidermis, but it may also lie immedi- 
ately beneath the epidermis or even at a considerable distance 
beneath the surface. In the latter case a number of such 
nuclei may be collected into a group which constitutes a gang- 
lion, and if the fibres run parallel in a bundle they form a 

361. The sensory cells of Arthropods are very much like 
those of nereis, but they do not always have the exposed proto- 
plasmic terminations. Instead, the fibre may end at the base, 
or in the axis of one of the cuticular sensory hairs mentioned 
above. (Page 146.) The hairs serve mechanically to trans- 



mit the stimulus to the sensory element but are not themselves 

362. The sensory elements found in the skin of Vertebrates 
are of various types which may be divided into two classes. In 
one class the fibre ends in a bushy system of branches which 
penetrate among the other normal elements of the surrounding 
tissues. In the other class the fibre may end with or without 

FIG. 76. The blue crab. View of the region around the mouth to show the 
groups of cuticular hairs, which are in large part sensory. X i 1/2. 

terminal branching, but in either case there are always some of 
the cells of the surrounding tissues modified to form a special 
stimulating organ. The first class, the free nerve terminations, 
are found chiefly in the deeper layers of the epidermis, though 
they may also be found in the dermis. The second class, which 
for want of a better name may be called sensory corpuscles, 
are found chiefly in the superficial and deeper layers of the 


dermis, though some are also found in the epidermis (in man 
only in the dermis). 

363. The nuclei of all sensory elements of the skin of verte- 
brates lie in the spinal ganglia and homologous ganglia of the 
cranial nerves. 

364. The sense organs just described are those which are 
generally distributed over the whole surface of the body. The 
senses to which they correspond are, in man, touch, cold, 
warmth and pain. Each of these senses, with the possible 
exception of pain, has its own set of sensory elements, although 
the correspondence between sense organs and senses has not 
yet been completely determined. How far these senses are 
differentiated in the lower animals is also not known. 

365. In the higher animals there are also deeper lying sense 
organs, which are located in the sub-cutaneous connective 
tissue, in the muscles and tendons and even in the mesentery. 
To these organs are ascribed a sense of weight and a sense of 
position, or attitude, of the member of the body with regard to 
the other members of the body. 


366. Besides the general sense organs described above, we 
find in all the higher animals special sense organs, which are 
developed in very limited regions of the body and which are 
often very complex in structure. These are the organs of 
taste and smell, which are stimulated chemically; the organs of 
hearing and equilibration, which are stimulated mechanically, 
and the organ of sight, which is stimulated by ether vibrations. 

367. From direct evidence we know little about the chem- 
ical senses of hydra, though as in the case of amoeba we may 
infer that the choice of food indicates a sense of this kind, but 
this evidence is by no means conclusive. In the sea anemone, 
however, it is found by experiment that the tentacles distin- 




guish food from other objects in such a way as to indicate a 
chemical sense. No specialized chemical sense organs have 
been distinguished. 

368. The earthworm is chemically sensitive over the entire 
surface of the body, but at the anterior end this sense is best 
developed. The function seems to be located in sensory cells, 
which occur in clusters, the clusters being distributed over the 
surface of the body in numbers which correspond approxi- 
mately to the sensitiveness of the region. 

369. The antennules of the crayfish are the seat of chem- 
ical sense. The sensory cells concerned are not specially 
modified, but their accessory terminal end-organs are peculiar 
club-shaped hairs, which are covered by an extremely thin 
cuticula. In insects, there is a differentiation of the chemical 
sense into an olfactory and a gustatory sense. The former is 
located on the antennae, and the sense organ consists of a flask- 
shaped cluster of sense cells which are exposed at the surface 
at the bottom of a pit in the cuticula. The organs of taste 
are similar, but are found on surfaces bounding the mouth 
cavity or on the mouth appendages. 

370. In arthropods we first observe a decided limitation of 
the chemical sense organs to the region of the mouth and in the 
air breathers, the first differentiation of the senses of taste and 
smell. In some fishes, organs of taste are found on the surface 

FIG. 77. Sense organs. A-E, General sense organs; F-H, organs of taste; 
7, olfactory organ; J-O, eyes. A, General sense organs of nereis; B, general 
sense organs of Arthropods; C, free nerve terminations in the epidermis of 
Vertebrates; D, sensory corpuscles in the dermis of Vertebrates; E, same, en- 
larged; F, diagram of human tongue showing distribution of fungiform (i) and 
circumvallate papillae; G, section through a circumvallate papilla showing 
position of the taste buds; H, section of a taste bud, showing two sensory cells, 
one supporting cell (c) and a nerve fibre (g) ; /, section of the olfactory epithelium; 
/, outline of a small "gliding worm" with two simple eyes; K, anterior end of 
another "gliding worm" with a number of simple eyes arranged along the edge of 
the body; L, eye of a "gliding worm" consisting of a single cell with a sensory brush 
partly surrounded by a pigment cell; M, eye of a snail (Patella); N, eye of 
nereis; O, part of N, on a larger scale, c, Supporting cell; Cu, cuticula; D, 
dermis; Ep., epidermis; g, fibre from a ganglion cell; h, sensory hair, or bristle; 
N.C., nerve cell; N.F., nerve fibres; 0. N., optic nerve; P, pigment; S, sense cells. 


of the body, more particularly in the region of the mouth, but 
with this exception we can say that for all vertebrates the organs 
of taste and of smell are limited to the surface of the cavities 
of the mouth and nostrils, respectively. 

371. The organs of taste are called taste buds, because the 
sense cells are grouped in small cask-shaped clusters. The taste 

buds may occur in various 
parts of the mouth, but in 
mammals they are chiefly 
found on the sides of the 
fungiform and circumvallate 
papillae (and the foliate 
papillae, where they are 
found) of the tongue. The 
bud consists of two kinds of 
cells, both very much elon- 
gated. One of these, the 
supporting cells, taper to a 
point at the free end while 
the deeper end is very irregu- 
lar in outline. The sensory 
cells are more slender and end 
at the free extremity in a short cuticular hair. At the other 
end they broaden out into a slight enlargement. They have 
no fibre processes. Nerve fibres from deeper lying nerve cells 
form a network of numerous branches, which enclose the bud 
and penetrate between the cells which compose it. 

372. In man there are four kinds of taste sensations: sweet, 
sour, salt and bitter. At the tip of the tongue sweet is most 
readily detected, sour along the edges, salt at the tip and edges, 
and bitter at the base of the tongue. We conclude, therefore, 
that these four sensations are yielded by as many different 
kinds of organs which, however, are not distinguishable 

FIG. 78. Antennae of a moth, Samia 
cecropia. A, Of male; B, of female. 
(From Folsom.) 


373. The sensations yielded by the olfactory organ are much 
more various than those of the sense of taste, but at the present 
no satisfactory analysis of olfactory sensations is possible. 
So far as can be seen under the microscope, however, the sense 
organs are very simple and all of the same kind. They present 
very much the same appearance as the sensory cells of the epi- 
dermis of nereis. The olfactory organ forms a small part of 
the mucous epithelium, lining the nasal cavity at its upper 
angle. The epithelium here consists of columnar cells of two 
kinds. The first are the sensory cells which are very slender 
and end in a group of six to eight short bristle-like tips. Below 
the nucleus the cell narrows to a very slender nerve fibre which 
goes to the brain. Between the sensory cells are the somewhat 
stouter ''supporting" cells. The superficial ends of these are 
quite regularly prismatic in form, but below the nucleus they 
are very irregular in form. A third type of cells, called " basal 
cells," form the deeper layer of the epithelium. 


374. Mention has already been made of the fact that sensi- 
tiveness to light is exhibited by protozoa, which have no sense 
organs. There are some protozoa, however, which have an 
"eyespot," a small speck of red pigment embedded in the pro- 
toplasm. These eyespots are found to be especially sensitive 
to light, and must, therefore, be regarded as an exceedingly 
simple type of light sense organs. 

375. Hydra is sensitive to light, but has no organs specially for 
light perceptions. In some other Ccelenterates, as some of 
the free swimming medusae, true light-sense organs are found. 

376. Among worms, again, sensitiveness to light does not 
always indicate the presence of well-defined sense organs 
especially constructed for this function. The earthworm is 
more or less sensitive to light over the entire body surface, but 



this is more marked at the anterior end of the body. It is 
possible that some of the sense organs scattered over the surface 
of the body are light-sense organs, but there is no direct evi- 
dence that such is the case. In many other worms, however, 
there are organs which are unquestionably eyes. They are 
usually on the head, but may be found else- 
where, and they vary greatly in number. 

377. One of the simplest of eyes consists of a 
single epidermal sensory cell, surrounded by a 
group of pigment cells. More often there are 
a large number of sensory cells in a compact 
group. When this is the case the epidermis at 
this point is greatly thickened, owing to the 
elongation of cells, and it is also usually con- 
cave toward the surface. The sensory cells 
taper below the nucleus into a slender nerve 
fibre which goes to a deeper lying ganglion. 
Above the nucleus the cell body is cylindrical, 
and from its end there project a number of 
slender bristle-like processes. There is always 
considerable pigment in such an eye. It lies 
either within the sensory cells themselves or else 
in the surrounding non-sensory cells. The 
cuticula over the eye is usually much thick- 
ened, and often has the double convex form of 
a condensing lens. Often, as in nereis, the sensory area sinks 
in so deeply that it approaches a complete sphere in form. 
It may then also separate entirely from the epidermis. 

378. In Arthropods the optic apparatus attains a much 
higher degree of functional perfection, and at the same time 
becomes much more complex. Its development, however, 
proceeds along very different lines. An eye similar in structure 
to that described for worms is found in many Crustacea, and 
the ocelli of insects are also much the same, but the com- 

FIG. 79. Dia- 
gram of an om- 
matidium. a, 
Cuticular cor- 
nea; b, corneal 
cells; c, cone 
cells; d, retinal 
cells; e, rhab- 
dom; /, fibres of 
the retinal cells. 



pound eyes of the higher groups of both Crustacea and Insects 
are of a different type. The compound eye is convex and it 
is made up of a large number of units called ommatidia. The 
surface of the cuticula is divided into numerous polygonal areas, 
the "facets," each of which corresponds to an ommatidium. 
The ommatidium is made up of the following cells: two super- 
ficial cells which secrete the lens-shaped cuticula; below these, 

FIG. 80. Horizontal section through the right human eye. a-p, Axis of 
vision; ac, central artery; ah, aqueous humor; b, blind spot; c, conjunctiva; 
ch, choroid layer of the eyeball; cl, crystalline lens; cmc, circular fibres of the 
ciliary muscle; c.m.r., radial fibres of the ciliary muscle; co, cornea; cp, ciliary 
process; cs, canal of Schlemm;/0, fovea centralis; on, optic nerve; os, ora serrata, 
the anterior limit of the sensory portion of the retinal layer; r, the retina; sc, the 
sclera; sh, sheath of the optic nerve; vh, vitreous humor. (From Galloway). 

four cells which form an egg-shaped lens, and below these, 
again, seven or eight sensory cells, so arranged as to form a 
single sensory unit. Around the whole is a cylindrical curtain 
of pigment cells. From the sensory cells, nerve fibres pass 
downward to a deeper ganglion. 

379. The sensory portion of all invertebrate eyes is developed 
from the epidermis, but the retina of the vertebrate eye and all 


the connected nervous elements are developed from the brain. 
The vertebrate eye is exceedingly complex, and only the more 
essential features will be called to mind: i. The sclera is a hol- 
low shell of approximately spherical form, composed of a thick 
and dense layer of connective tissue. It is the protective and 
supporting framework of the eye. The cornea is the trans- 
parent, more convex portion of the sclera on the side where 
the light enters the eye. 2. The choroid layer is a layer of 
blood vessels and capillaries, which lines the inner surface of 
the sclera. In front it forms the iris, and an opening in the 
latter is the pupil. 3. The retinal layer is double. Against 
the surface of the choroid layer there is a layer of pigment cells, 
which extends from the point where the optic nerve enters the 
eye to the pupil. The retina proper is the innermost layer and 
extends from the optic nerve to within about 60 of the centre 
of the pupil, where it thins out into an endothelium, and as 
such, continues on to the edge of the pupil, where it merges into 
the pigment layer. 

380. The nervous elements of the retina are arranged in 
three layers: i. The sensory layer, proper, is composed of two 
types of cells, rods and cones, as they are called. The rods are 
much more numerous than the cones, except at the point of 
most distinct vision the fovea centralis where the rods are 
entirely wanting. The "rods" consist of a slender cylinder, 
which tapers at one end into a short fibre. The latter is more 
or less beaded and ends in a small knob. The cones are shorter 
and thicker than the rods and, as the name signifies, are conical 
in form. From the base of the cone a rather stout fibre pro- 
ceeds, but ends shortly in a broad disc. The nuclei of the 
" cones" are rather large and located at the base of the cone. 
The nuclei of the rods are smaller and lie somewhere along the 
course of the fibre. The cylindrical and conical portions of 
the rods and cones, respectively, project into the pigment layer 
in such a way that their ends are completely surrounded by 



processes of the cells of the pigment layer. The fibre ends of 
the rods and cones project toward the centre of the eye. 2. 
The bipolar cells of the second layer are short nerve cells which 
end at either extremity in a tuft of branches. They seem to 
connect the first and third layers. 3. The ganglion cells of the 

FIG. 81. Diagram showing some of the retinal elements. Layer i is 
nearest the centre of the eye and consists of nerve fibres (/) which enter the 
optic nerve at the blind spot; 2, the ganglionic-cell layer, made up of nerve 
cells from which the fibres (/) arise; 3, the inner molecular layer made up of 
the minute branches arising from the cells of layers 2 and 4; 4, the inner nuclear 
layer, containing the nuclei of the short elements which connect layers 3 and 5; 
5, the outer molecular layer which is similar to layer 3; 6, the outer nuclear layer 
contains the nuclei of the rod and cone elements; 7, the layer of rods (r) and cones 
(c); 8, the pigment layer. The rods and cones are the sensory elements. They 
project into the pigment layer. (From Galloway.) 

third layer are large cells with a bush of protoplasmic processes 
and a long fibre. The fibres form a layer on the surface of the 
retina next the centre of the eye. They all converge to the 
point where the optic nerve enters the eye. It is these fibres 
with their medullary sheaths that constitute the optic nerve. 



381. Just inside the pupil lies a double convex lens. It 
originates from the epidermis by an infolding. It is the densest 
organic structure of the body. It is fibrous in structure and of 
glassy transparency. The lens is enclosed in a capsule, which 
is attached by means of fibres to the muscular ciliary body, a 
portion of the choroid layer. 

382. The large central cavity of the eye is filled with a trans- 
parent jelly, the vitreous humor. The smaller space in front 
of the iris contains a more fluid, aqueous humor. 

FIG. 82. Diagrams to show how the concave and convex arrangement of 
the sensory elements in invertebrate eyes serves to indicate the direction of 
the light rays. A, The concave eye, like that of Patella; B, the convex eye, 
like that of the compound eyes of Arthropods. 

383. Vision. The simplest type of eye described doubtless 
enables the possessor to distinguish more readily differences in 
the intensity of light, such as a passing shadow. The animal 
would not be able, however, to distinguish one object from 
another by its form, since the eye is not so constructed as to 
form an image. Where there are a number of such eyes so 
placed on the body that they "look" in different directions, the 
stimulation of one more than another would be an indication 
of the direction of the source of light. In the slightly more 
complex eye, where the elements are arranged radially on a 
concave surface, there is formed a crude image, because the 


arrangement of the pigment and the sensory elements deter- 
mines that each element is stimulated from a particular direc- 
tion. The image in this case is formed by projection. If 
there is a cuticular lens it is too close to the retina to form an 
image; it only serves to concentrate the light. 

384. When the sensory elements are arranged on a convex 
surface as in the insect eye, an image is also formed by pro- 
jection, but in this case the image is erect while in the concave 
eye it is inverted. However, the cuticular lens and the cone 
of the ommatidium are so placed that an image is formed in 
the plane of the retinal cells. There is, therefore, a combina- 
tion of image by projection and image by refraction. This 
type of eye is comparatively efficient. Form is distinguished 
with considerable detail, and colors are recognized, but there 
is still a deficiency which makes the insect eye decidedly in- 
ferior to the vertebrate eye. There is no provision for focusing. 
It is possible that the great depth of the sensory element (the 
rhabdom) is in some measure a compensation; the multiplicity 
of eyes is another. In some cases a single eye is so constructed 
that one part is adapted for far vision, the other for objects 
near at hand. 

385. In some Vertebrates (fishes) the eye is focused by mov- 
ing the lens toward or away from the retina. A more refined 
method is adopted by the higher Vertebrates. The lens is 
elastic and is continually flattened somewhat by the pressure 
of the capsule on the anterior and posterior surfaces of the lens. 
By the contraction of the ciliary muscles this pressure is some- 
what removed and the lens, by elasticity, assumes a more con- 
vex form. 

386. In many eyes the quantity of light admitted to the 
sensory elements is controlled by movements in the pigment 
cells. When the light is too intense the pigment advances 
and cuts off some rays. In weak light the pigment recedes, 
thus admitting a broader beam of light. This adjustment is 


well developed in the insect eye. In the vertebrate eye this 
adjustment is supplemented by a change in the size of the pupil. 
A circular muscle in the iris causes the pupil to contract while 
a set of radial muscles cause it to expand. 


387. Statocysts. Many of the lower animals have been 
credited with a sense of hearing, but it is very doubtful whether 
any aquatic invertebrate has really an organ for perceiving 
sound. That many aquatic animals may "feel" and respond 
to vibrations set up in the water is quite probable. But this 

may be due to the stimulation of 
other organs, such as the tactile 
sense organs. The organs found 
in jellyfishes, worms, Crustacea, 
and many other aquatic inverte- 
brates, which have been called 
"ear sacs," are well understood 
and are more properly called 

FIG. 8 3 .-Statocyst of a Mol- 388. In hydra there are no 

hisc. , Nerve; o, otolith; s.c., statocysts, nor are they found in 
sensory cells. (From Galloway, 

after ciaus.) any other fixed forms. In the 

hydromedusae, however, they are 

very common. They consist, typically, of a deep sack-like 
depression of the ectoderm, which contains sensory cells and 
a statolith. The sack may be open or closed, but in either 
case is filled with a fluid. The sensory cells are provided 
with bristle-like processes which project into the cavity of 
the statocyst. The statoliths are heavy concretions of inor- 
ganic matter which stimulate the sensory cells by contact 
with the bristles. When the animal turns over in swimming, 
the statoliths, by their weight, always settle to the lower side 


of the statocyst and stimulate the cells in that region. By 
this means the organism is informed of the orientation of its 
body in space. 

389. No statocysts are found in either nereis or the earth- 
worm, but they are present in some other Annelids. 

390. On the upper surface of the basal portion of the anten- 
nules of the crayfish there is a small opening which leads into a 
statocyst. The inside of the sack is lined with sensory hairs, 
upon which rests the statolith. In this case the statolith is 
composed of grains of sand cemented together by a secretion 

FIG. 84. Diagram of the internal FIG. 840,. Diagram of the laby- 

ear (labyrinth) of one of the lower rinth of a mammal showing the 
vertebrates, u, Utriculus with three cochlea, 
semicircular canals; s, sacculus; /, 

of the epidermis. The sand is introduced into the sack by the 
animal itself after each ecdysis, for the lining of the sack " sheds" 
like the remainder of the cuticula, and its contents are cast 
out at the same time. 

391. Statocysts are practically wanting in insects. 

392. The Vertebrate Organ of Equilibration. The internal 
ear of vertebrates consists of a membranous sack, the labyrinth, 
which is lined internally with a layer of cells of ectodermal origin. 
At certain places in this lining there are groups of sensory cells, 
which have a close resemblance to the sensory cells of the stato- 
cysts just described. The labyrinth is filled with a fluid and 


contains a large calcareous concretion, the "ear stone," or 
numerous smaller particles which are called ear sand. 

393. The labyrinth of the round-mouth eels is a simple 
ovoidal sack, but in the higher fishes the sack is partly divided 
into two chambers, a utriculus and a saculus, and connecting 
with the utriculus are three semi-circular canals, two of which 
are in vertical planes but at right angles to each other, while 
the third canal is horizontal. 

394. In the higher vertebrates the utriculus with the three 
semi-circular canals, and the sacculus are also found, and, in 
essential features, the same as in the higher fishes. 

395. The function of this part of the vertebrate ear is the 
same as that served by the statocyst of the invertebrates. It 
has nothing to do with hearing. It is an organ of orientation 
and equilibration. If the organ is destroyed or the nerve 
leading to it severed, the animal has difficulty in maintaining 
its normal upright position. A fish, for example, which has 
lost the use of this organ no longer swims in its normal way. It 
turns over and over, or may swim with its back downward. 

396. The Auditory Organ. The sense of hearing seems to be 
primarily developed in connection with voice, and it is doubtful 
whether there is any species in which one occurs without the 
other. Within the class Insecta we find the only invertebrates 
having sense organs for the perception of sound, and the species 
in which they occur best developed are our singing insects, the 
grasshopper, katydid, cricket, and cicada (" locust," " harvest 
fly," "jar fly"). The singing in these cases is usually done by 
the male, and is intended for the "ears" of the female. Many 
sounds are produced by animals, which are accidental, and can- 
not be called voice, as in most cases the buzzing produced by 
the wings in flight. At the same time the buzz of the wings 
may, in some cases, be used as a means of communication be- 
tween individuals, in which case it would have to be regarded 
as voice. On the other hand, sound vibrations may be per- 

THE EAR 169 

ceived by sense organs other than that of hearing, hence a 
response to a sound is not necessarily an indication of the 
presence of an organ of hearing. It must also be kept in mind 
that other animals may make and hear sound vibrations of so 
high a pitch as to be inaudible to the human ear. 

397. The ear of the grasshopper is called a tympanum, 
because of its resemblance to a drum head. It is, in fact, a 
thin membrane stretched over a large respiratory cavity, and 
is located on the side of the first abdominal segment. The 
katydid and cricket also have tympanums, but they are located 
on the tibia of the anterior legs. But in the essential points 
these organs are similar in structure. The sensory apparatus 
consists of groups of sensory cells, intimately connected with 
the inner surface of the tympanum. The tympanum is highly 
responsive to vibrations of the air, and by its own vibrations 
the connected sensory cells are stimulated. 

398. At one side of the sacculus, in frogs and reptiles, there 
is a small pocket which is not found in fishes. In birds this 
pocket becomes a long tube, and in mammals it is very long 
and coiled. This is the cochlea and is the true organ of hearing. 
On one side of the cochlea the lining epithelium is composed of 
peculiarly arranged columnar cells, which form what is known 
as the organ of Corti. The cochlea is filled with a fluid, endo- 
lymph, like the other parts of the labyrinth. In a cross section 
of the organ of Corti there are several supporting cells and about 
four sensory cells, but in a longitudinal section there would be 
from 4,000 to 5,000 sensory cells, covering a space of more than 
25 mm. The sensory cells are rather stout and rounded at 
the lower end. At the free end they each bear about twenty 
rod-like processes, which project into the endolymph. This 
organ rests on a membrane of fibres (the basilar membrane) 
which stretches across from the bony wall of one side to that of 
the other. Above the sensory cells, suspended in the endo- 
lymph, is a thick cuticular membrane (membrana tectoria), 


which almost touches the processes of the sensory cells. This 
membrane is free at one edge, but attached at the other to a 
non-sensory portion of the cochlea. The nerve fibres supplying 
this organ end in free nerve terminations around the sensory 
cells. The basilar membrane becomes wider toward the apex 
of the cochlea, and the fibres of the basilar membrane become 
correspondingly longer. 

399. The entire labyrinth lies in a cavity of approximately 
the same shape, in the petrosal portion of the temporal bone. 
This is the bony labyrinth. It is considerably larger than the 
membranous labyrinth, and the space between is filled with 
perilymph. A small opening in the wall of the bony labyrinth 
is covered by a membrane, and a small, movable bone, the 
stapes. By the vibrations of these parts the perilymph is 
disturbed and through it the fibres of the basilar membrane and 
thus the cells in the organ of Corti are stimulated. It is 
supposed that the difference in the lengths of the basilar mem- 
brane fibres corresponds to differences in the lengths of sound 
waves, and that, therefore, sounds of a given pitch stimulate 
only that part of the organ of Corti in which fibres of a cor- 
responding length occur. 

400. Certain accessory organs, by which the sound waves 
in the air are transmitted to the fluids of the labyrinth are 
found in all animals having a well-developed sense of hearing, 
and the condition of these organs is a very good index as to the 
degree of perfection of the sense itself. 

401. The auricle, or shell, of the outer ear is found only in 
Mammals. Its function is manifestly to gather the sound 
waves and direct them to the auditory meatus, the tube which 
leads to the ear drum. The external auditory meatus is also 
found in Birds and some Reptiles. But in the frogs the ear 
drum is on a level with the surface of the head. Some Reptiles 
(snakes), some Amphibia (salamanders) and Fishes have 
neither outer nor middle ear. 



402. The ear drum is a tightly stretched membrane, which 
is so constructed that it vibrates equally well with sounds of 
different pitch. 

403. Between the ear drum and the inner ear there is a small 
cavity which communicates with the pharynx through the 

FIG. 85. FIG. 87. 

FIG. 85. Cross section of one turn of the cochlear spiral as it lies in position 
in the long labyrinth. The organ of Corti (above the letter C) rests on the 
basilar membrane and nerve fibres run out to the spiral ganglion N. 

FIG. 86. Part of the organ of Corti, to show the sensory cells and the nerve 
fibres leading to the spiral ganglion. 

FIG. 87. Diagram of the middle ear of a mammal. E, External auditory 
passage, ending at the ear drum; /, internal ear; M , middle ear, opening into the 
pharynx by the Eustachian tube E.T.; i, malleus; 2, incus; 3, stapes, fitting 
into the oval window. 

Eustachian tube. This cavity is the middle ear. Its most 
important parts are three small bones, the hammer, the anvil 
and the stirrup, through which the vibrations of the ear drum 
are transmitted to the perilymph. The hammer is attached 
to the ear drum, the stapes fits over the opening in the bony 
labyrinth, and the two are connected by the anvil. In Birds 


and those Reptiles and Amphibia which have an ear drum the 
bones of the middle ear consist of one piece only, the columella. 

404. Caution. Our knowledge of the senses of animals is still 
far from complete. Since we cannot experience the sensations 
of another human being we can only, in a general way, infer what 
they are by supposing them to resemble our own. Such an 
assumption, with regard to the lower animals, is of little value. 
We have senses of which we are unconscious (the sense of the 
organ of equilibration and the senses of the deep lying organs 
mentioned above), and the lower animals may have senses 
which we do not have. The lateral line of Fishes, for example, 
is a system of sense organs by which the animal is informed of 
movements of the water. Many other sense organs have been 
found whose function is unknown. 

405. Function of the Senses. Concerning sense organs in 
general, it may be said that animals are provided with sense 
organs for perceiving those changes in the environment which 
might operate either to the advantage or detriment of the or- 
ganism, and to which the organism is capable of making an ef- 
fective response. Within the meaning of the term as used here, 
we are insensible to those constant elements- of the normal at- 
mosphere, oxygen, carbon-dioxide .and nitrogen, although 
oxygen is absolutely and constantly necessary to life, while 
carbon-dioxide in large quantities is fatal. Nor do we possess 
organs for dectecting changes in the force of gravity, of at- 
mospheric pressure or of electrical conditions, for the evident 
reason that either the welfare of the organism is not affected 
by the changes which normally occur or else that no effective 
response is possible. 


406. When an animal is sufficiently stimulated a response 
occurs. This is usually in the form of a contraction or expan- 


sion or a combination of both. Very often, however, response 
takes the form of glandular activity. Some times light is 
produced and some times an electrical discharge. The latter 
responses are relatively rare. We will now 
consider the organs of response; and first the 

407. When an expanded amoeba is strongly 
stimulated it contracts into a spherical mass. 
How this is done we do not know. It is a 
property of undifferentiated protoplasm in 
which no contractile elements of any kind can 
be distinguished. In some other protozoa 
(paramcecium and stentor) there are distinct 
contractile elements in the form of slender 
fibrils (myonemes), which traverse the ecto- 
plasm in a longitudinal, slightly spiral direc- 
tion. By their contraction they also cause the 
animal to assume a more nearly spherical form. 
Such cells contract more energetically than 
does the amoeba. 

408. In hydra, the contractile fibrils of each 
cell are grouped into a bundle, the muscle 
fibre, which is much longer than the body of 
the cell and projects on either side. This gives 
the cell the form of a T with a very short stem, 
representing the cell-body and the cross bar 
representing the fibre. The fibres of the ecto- 
derm cells run longitudinally, while those of 
the entoderm run circularly around the body. 
In this case part of the cell remains undiffer- 
entiated and continues to form part of an 
epithelium. This condition is also met with in others of the 
lower phyla, but in the Annelids and higher forms the differentia- 
tion proceeds farther and involves the entire cell. So we find 



that besides the epithelial layers which cover the exterior of the 
animal and line the internal cavities, there are also other tissues 
like the muscles which lie between the ectoderm and entoderm. 
These other tissues constitute a third layer, which is called 


FIG. 89. Structure of the body-wall of hydra. A, Part of a cross section 
of the column; B, the region between ectoderm and entoderm, more highly 
magnified to show the longitudinal and circular muscle fibres; C, diagram of 
an ectoderm cell with a muscle-fibre process. Ect, ectoderm; En, entoderm; 
/, muscle fibre; N, nematocysts; s, supporting layer between ectoderm and 

mesoderm (in the embryonic stage of development) . This layer 
is entirely wanting in hydra and is not well developed in any 
of the Ccelenterates. 

409. The muscles of the Annelids are composed of fibres, 
and each fibre consists of a bundle of muscle fibrils. Each 

FIG. 90. A branched muscle fibre from the wall of a blood-vessel (Nereis). 

fibre has a nucleus and represents an elongated cell. The fibres 
taper to a point at each end and are arranged parallel in 
masses called muscles. In the worm the principal muscles are 
arranged in two sets. Just beneath the epidermis there is a 
thin layer which runs circularly around the body, and beneath 



this there are four large masses which run longitudinally, two 

on the dorsal side and two on the ventral. 
410. The muscle systems and the integument form a hollow 

cylinder, which is closed at both ends and filled with a fluid, 

the body fluid. This constitutes a locomotor mechanism, 
which operates as follows: When the 
circular muscles contract the body be- 
comes more slender, and must, there- 
fore, elongate, which causes the longi- 
tudinal muscles to expand. When the 
longitudinal muscles contract the body 
is shortened and must become corre- 
spondingly thicker, which causes the 
circular muscles to expand. The ac- 
tivity of a muscle is always expressed 
by contraction. When it expands it 
is passive, the action being due to the 
contraction of other muscles. The 
worm is provided witri groups of 

FIG. 91. Plain muscle 
fibres. n, Nucleus; p. 
protoplasm; p', muscle 

FIG. 92. Three muscle cells of nereis in cross sec- 
tion. The dots on the periphery of the cell are the 
muscle fibrils in cross section. 

bristles (setae) on either side of each segment. These bristles 
can be set forward or backward by means of small ifluscles 
connected with them. The setae prevent slipping ofthe body 
in the direction in which they are set. If now the setae are set 
backward and the circular muscles contract, the anterior end of 
the animal moves forward, the posterior end remaining fixed. 
Then, when the longitudinal muscles contract, the posterior 
end moves forward, and the anterior end remains fixed. A 



repetition of these events causes another hitch forward, and so 
the worm progresses by a process called inching. 

411. If the setae are set forward and the muscular contraction 
repeated in the same sequence as before, the animal inches back- 
ward, and practically with equal facility. This is the chief 
method of locomotion of the earthworm, but nereis, by wave- 
like contractions of the longitudinal muscles, alternately right 
and left, causes a serpentine movement of the body by which 
it creeps along. A similar mode of contraction, alternating 
between the dorsal and ventral muscles, causes an undulatory 
up and down motion by which nereis swims. The leech swims 
in the same way. 

412. In Arthropods, the method of locomotion is totally 
different. Here the animal, both body and appendages, is 
encased in a series of rings or cylinders composed of the stiff 
cuticula and permitting of practically no change of form within 
the individual segments. However, the segments are flexibly 
connected with each other and provided with muscles attached 
in such a way that the segments may be moved with respect 
to each other. Thus the ambulatory appendage of the cray- 
fish consists of six segments, connected with each other and with 
the body by six hinge-like joints. The muscles lie inside the 
cylindrical segments of integument to which they are attached 
and extend across the joint from segment to segment. When 
a muscle contracts, it flexes the appendage at the joint between 
the two points of attachment. The muscles are arranged in 
pairs at each joint, and the two muscles of a pair move the ap- 
pendage in opposite directions. The appendages operate 
purely as levers, and locomotion is wholly due to leverage action. 
This is true also of the action of the tail fin, which by its power- 
ful strokes causes the body to shoot backward. The wings 
of insects likewise operate as levers. In these cases the ful- 
crum of the leverage action is the water or air instead of the 


413. Locomotion in Vertebrates is similar to that of Arthro- 
pods in principle, with this important difference: The levers 
occupy the axis of the appendage while the muscles are attached 
to the surface and lie outside. A mechanical advantage is here 
obtained by the greater flexibility of the joints. The hinge 
joint, which is the only one possible in Arthropods, permits of 
motion in one plane only, while the ball and socket joint, which 
is found at many points in the vertebrate skeleton, gives uni- 
versal motion. 

414. The muscular tissue of the body- wall and the organs of 
locomotion, is composed of fibres of a complex structure. 
Almost the entire substance of the cell is transformed into 
muscle fibrils, of which there are a large number. There is 
also a fibre sheath which binds the fibrils together, and among 
the fibrils are a number of nuclei. This type of fibre is, there- 
fore, not a uninuclear cell. The most striking characteristic of 
these fibres is the banded appearance which they present under 
the microscope. The light is affected differently at different 
points in the fibre so that some appear light and others dark. 
These points alternate regularly and give the fibre the appear- 
ance of being crossed by alternating dark and light bands. Such 
muscular tissue is called cross striped or striate and distinguishes 
the skeletal muscles of Vertebrates and Arthropods from the 
muscles of Worms and most other invertebrates. The heart of 
Vertebrates is also composed of striate muscle, but the muscles 
of the digestive tract and many other parts of the body are more 
like those of the Annelids. They are called smooth muscle 
fibres. Generally the cross-striped muscles are more quick and 
vigorous in action than the smooth muscle fibres. See Fig 101. 


415. Between the ectoderm and entoderm of hydra there is 
a thin layer called the supporting lamella. It is secreted by the 
cells of the ectoderm and entoderm, and is, therefore, not a 



cellular layer. It is of a firm gelatinous substance, which 
probably adds to the rigidity of the body. In other Ccelenter- 
ates this layer is much thicker, and especially in jelly fishes it 
forms the major portion of the mass of the animal. In these 
cases it may contain cellular elements of various kinds, which 
have moved into it from the ectoderm and entoderm. 

416. Worms generally have no skeleton, but special tissues 
are sometimes developed in connection with certain organs. 
The nervous system of nereis is enclosed by a thick covering 
of a non-cellular connective tissue. 

417. The chitinous cuticula of the Arthropods forms a 
highly efficient exoskeleton. To this the muscles and other 
internal organs are attached through the medium of the epi- 
dermis and basement membrane. The latter is a thin non- 
cellular layer secreted by the epidermis on its inner surface. 

418. In the Mollusca, also, the exoskeleton is usually ex- 
tremely well developed. In this case it consists chiefly of thick 
layers of limey salts, deposited between the cuticula and the 
epidermis. This " shell" is formed only by the epidermis of a 
special fold of the skin called the mantle. 

419. An exoskeleton is rather exceptional among Vertebrates. 
The scaly covering of the body of most Fishes is an excellent 
protective structure, but is rather too flexible to be called an 
exoskeleton. In the head region, however, the scales are often 
so intimately united as to form a real supporting shell. In 
special cases this shell is extended over a large portion of the 
body, as in the gar pike and trunk fishes. 

420. The shell of the turtles is composed partly of the ex- 
panded ribs of the endoskeleton and partly of bony plates formed 
in the skin. The whole of the bony portion is covered by horny 
plates developed by the epidermis. 

421. The jointed shell of the armadillo is made up of bands of 
dermal bone, while the covering of the scaly ant-eater is com- 
posed of large, horny scales. 



422. A well-developed endoskeleton is found only in 
Vertebrates, but the notochord of the lancelet may be regarded 
as an endoskeleton of the simplest form. It is simply a rod of 
large turgid cells with strong cell membranes. This rod ex- 
tends lengthwise of the body, immediately under the spinal 
cord. It is also found well developed and functional in the 
round-mouth eels, but in other fishes the vertebrae are 

FIG. 93. Outline drawing of the lancelet (Branchiostoma) to show the position 
of the notochord (N.C.) and the spinal cord (S.C.). 

formed around it and take its place, though traces of it remain 
in the adult. In the higher Vertebrates it is formed in the 
embryo, but all evidence of it disappears with the development 
of the spinal column. 

423. The first evidence of a true internal skeleton occurs 
in the round-mouth eels. Here small pieces of cartilage are 
formed around the notochord and spinal cord. These pieces 
do not unite to form vertebrae, but they are arranged in a series 
segmentally. Beneath the brain and around the pharynx a 
large number of similar cartilages occur. In the sharks and rays 
and some other fishes like the sturgeon, the skeleton is also 
cartilaginous, but better developed. There is a continuous 
column of vertebrae and a skull. 

424. In the higher Fishes and all the higher classes of Verte- 
brates the skeleton is also, at first, cartilage, but this is gradually 
transformed into true bone. Certain parts of the skeleton 
remain cartilaginous throughout life, even in the highest forms. 

425. Cartilage is composed of cells which secrete an ex- 
tremely firm gelatinous substance. This substance is secreted 



in such large quantities that the cells themselves come to lie 
far apart in the jelly. Cartilage is sometimes semi-transparent. 
Sometimes it contains fibres, and very often it is hardened 
by deposits of lime salts. 

426. When true bone is formed, it either takes the place of 
cartilage or else is formed where no cartilage had previously 
existed. In the first case, the cartilage is first dissolved and 
in its place solid masses of lime salts are laid down, layer upon 

layer, by special bone-forming 
cells. Some cells become em- 
bedded in the bone and these 
are connected with each other 
by slender protoplasmic threads. 
The cells and their connecting 
threads form the lacunae and 
canaliculae of the dry bone. 
Around the blood vessels the 
FIG. 94. Cartilage cells lying singly, bone is deposited in concentric 

or in small groups of two or three, in lo vpr o K 1lt P lcp W V,p rP tVp lavpr* 
the cartilage jelly which is secreted by ia y ers > c ut eisewn< 3 lay 61 

them. are parallel with the surface of 

the bone. The layers are called 

lamellae; the spaces occupied by the blood vessels, Haversian 

427. When the bone does not take the place of a cartilage, 
it is formed in connective tissue. Such bone is called mem- 
brane bone. 

428. The skeleton is bound together by bands of exceedingly 
strong, elastic connective tissue called ligaments. They are 
found at the joints, binding bone to bone, so as to keep each in 
its place. They are not connected with the muscles. 

429. The muscles are sometimes connected directly with the 
bones, sometimes indirectly through the medium of tendons, 
which are bands of inelastic connective tissue. The muscles 
which produce motion at a given joint must be connected 



across the joint from bone to bone. But a mass of muscle 
around a joint would impede motion. The tendon of a muscle 
is not nearly so thick as the muscle, consequently where free- 
dom of motion is important, the muscle is frequently connected 

FIG. 95. Bone, in cross section. In A the surface of the bone is uppermost; 
B, an Haversian system more highly magnified, h, Haversian canal; I, lacuna 
the lacunae connected by canaliculi; a, artery; v, vein; la, bony lamella. (From 

at a distant point and only the tendon crosses the joint. Note, 
for example, that the muscles of the fingers are located in the fore- 
arm, and the tendons can be traced across the wrist and knuckle 
joints. The proximal point of attachment of a muscle is its 

FIG. 96. Connective tissue, showing fibrous structure and a few scattering cells. 


the distal point of attachment the " insertion." 
The middle, thicker portion of a typical muscle (like the 
biceps) is the " belly." The muscle is composed of muscle 
fibres, arranged in bundles. The fibers of each bundle are 


bound together by connective tissue, and the bundles are bound 
in the same way to form the muscle as a whole. 

430. When a muscle contracts it becomes shorter but pro- 
portionally thicker, so that its volume is not changed. At 
the time of contraction it also undergoes electrical and chemical 
changes, and heat is evolved. These subjects are discussed 

431. The cause of a muscular contraction is in every case a 
stimulus. Chemical stimuli, like salts and acids, applied to the 
muscle, will cause a contraction. The electric current will do 
the same. But the normal stimulus for the body musculature 
of the higher animals is a nerve impulse originating in some 
other part of the body. The origin of this impulse is in every 
case to be traced to some peripheral sense organs. In some 
cases the impulse may seem to arise in the central nervous 
system, but a careful analysis will show that even in these 
cases the impulse can be traced backward to some sense organ. 

432. The glands, luminescent organs and electrical organs 
are discussed elsewhere. 

433. We will next consider the means by which connection 
is made between the sense organs and the organs of response. 


.434. In no respect do the highest animals diverge so. greatly 
from the lowest as in the way they respond to stimuli. This 
difference is due, not so much to the differences between the 
sense organs or the organs of response, as to the way in which 
the two sets of organs are related. In amoeba the organs of 
sensation and response are identical, and no system of communi- 
cation is required, but in mammals the organs of communication, 
the brain and spinal cord, exceed in complexity all other 
organs of the body combined. 

435. We have seen that the muscle fibres of hydra are parts 


of cells which are otherwise undifferentiated and exposed at the 
surface. Here, then, the stimulus received by one part of a cell 
is transmitted to another part of the same cell. In other 
Ccelenterates, some of these epithelial 
cells are prolonged at the deeper ex- 
tremity into slender fibres, which are 
regarded as nerve fibres instead of 
muscle fibres. In this case the en- 
tire cell is nervous in function, it 
receives and transmits stimuli. 
Again, cells are found which do not 
reach to the surface of the epithelial F IG - ^--Diagram of nerve 

cells found beneath the ecto- 

layer to which they belong, and are, derm of a jelly-fish, 
therefore, probably not sensory. 

These cells, however, are multipolar nerve cells. They serve 
only to transmit stimuli. We have, therefore, three types of 
nerve cells: 

1. Sensory Transmitting Motor. 

2. Sensory Transmitting. 

3. Transmitting. 
While in the taste buds are found: 

4. Sensory. 

436. The first class is an extremely low type of differentiation, 
and is not found in the sensory-motor mechanism of the higher 

437. In the jelly-fish many cells of type three are found, 
and they are largely grouped in a circle of ganglia around the 
edge of the umbrella. This circle of ganglia constitutes a simple 
central nervous system, arranged with reference to the radial 
organization of the animal. 

438. In Annelids, the central nervous system is bilaterally 
arranged. There is a double ganglion in each segment, and all 
the ganglia are connected longitudinally by a double nerve. 

1 84 


This chain of ganglia lies on the ventral side, just on the inside 
of the epidermis. The fibres of the sensory cells enter the 
ganglia through the paired lateral nerves, which spring from 
each ganglion and pass out to all parts of the corresponding 

FIG. 98. 

FIG. 99. 

FIG. 98. Diagram of the nervous system of nereis. A, The brain, ventral 
nerve cord, and the five nerves of a metamere; B, diagram of a parapodium to 
show the chief branches of nerve II. Br, Brain; C, circum-cesophageal con- 
nectives; E, eye; N, nerves; Oe, oesophagus; Par, parapodium;, parapodial 
ganglion; S.g., segmental ganglion; V.N., ventral nerve cord. 

FIG. 99. Diagram to show the relation of sensory fibres (S.F.), motor fibres 
(M.F.) and association fibres (A.F.) in a simple type of central nervous system. 
Ep, epidermis; Gl, ganglion; M , muscle; V.N.C., ventral nerve cord. 

segment. The ganglia contain two classes of cells: i. Those 
which send fibres up and down the chain to other ganglia, but 
do not pass out of the central system, and 2, those which send 
fibres out to the other organs through the lateral nerves. In 


addition to the ventral nerve chain, the worm has a ganglion 
which is not duplicated and which serves as a centre for the 
entire system. This ganglion is located in the cephalic lobe, 
on the dorsal side of the oesophagus. It is called the brain or 
supra-cesophageal ganglion. It is connected with the ventral 
chain by a pair of connecting nerves, which pass around the 
oesophagus and unite with the first ventral, or sub-cesophageal 
ganglion. The nerves from the eyes, tentacles, cirri and other 
special sense organs of the head region, connect with the brain. 
The central nervous system of Worms is usually sharply marked 
off from surrounding tissues, and is enveloped in a special 
protective connective tissue. 

439. The nerve elements are well differentiated and fall 
naturally into three distinct classes: i. The sensory cells 
(see page 360), which are called receptors, always lie outside the 
central nervous system, and send a fibre into the ganglion of the 
corresponding segment. 2. The connecting fibres, called also 
association fibres, lie wholly within the central nervous system 
and serve as a connection between the cells of the first and third 
classes. The cells of these fibres lie in the ganglia. 3. Cells of 
the third class lie in the ganglia, but send fibres out to the mus- 
cles, glands and other organs of response. These cells are 
called effectors. 

440. Where the receptor fibres end in the ganglia, they 
divide into a tuft of small branches, which end in contact with 
similar branches of other elements or with the body of another 
cell. The fibres of related cells are also often intimately con- 
nected where no branches occur. In this way, physiological 
connection is made between nerve cells and the stimulus 
transmitted from one to another. The effectors, where they 
end on muscles or glands, also break up into numerous small 
branches, which terminate in small disc-like enlargements in 
contact with the response organs. 

441. The mechanism of response is then as follows: When a 



sense organ is sufficiently stimulated a process is set up which 
travels along the receptor element to the central nervous 
system. Here connection may be made directly with an effec- 
tor element, and the stimulus pass out on another fibre to a mus- 
cle or other response organ and bring the latter into action. 
Or, the stimulus may be taken up by an association element, 

FIG. ioo. A photomicrograph of a section through the ventral edge of the 
brain of the crayfish. The black line represents the plane of symmetry. On 
either side of it are groups of ganglionic cells and, farther out, part of the circum- 
cesophageal connectives in cross section. Among the larger cells may be dis- 
tinguished at least six pairs of cells. The two cells of each pair are alike in 
size and exactly symmetrical in position and presumably their functions are 
identical with regard to the corresponding side of the body. This example 
seems to indicate that within the nervous system differentiation may extend 
to individual cells. 

and through it transferred to an effector of the same or a distant 
ganglion. Thus a response may occur in a distant portion of 
the body. A stimulus originating in the periphery may first 
pass to the brain, and from there return to the organs of response. 
442 . The superiority of the nervous system of the worm over 
that of the Ccelenterate is evident in the more complete dif- 


i8 7 

FIG. 101. Nerve cell and striate muscle fibre. A, Nerve cell; g, the body of 
the cell a ganglionic cell; d, dendron or dendritic process; a.x., axis cylinder 
process or nerve fibre. At n.f. the fibre is covered by a medullary sheath; 
n.m., the ending of the nerve fibre on a muscle fibre. D, Part of nerve fibre 
highly magnified; a, axis cylinder; m, medullary sheath; s, sheath of Schwann; 
n, node; m.f muscle fibre; /, muscle fibril. B, Muscle fibril highly magnified; 
C, the same in contracted condition. (From Galloway.) 


f erentiation of the elements, but the more significant difference 
is the centralization. The brain of the worm has no counter- 
part in the medusae where there are a number of centres of co- 
ordinate rank. 

443. The central nervous system of Arthropods is, in most 
respects, much like that of Worms. The most significant 
difference is the better development of the brain, which results 
largely from the better development of the special sense organs. 
The differentiation of body segments and of the appendages 
also makes possible a much greater number of movements, and, 
therefore, demands greater complexity in association elements 
of the nervous system. Besides, the nervous system is itself 
subject to independent differentiation, and this manifests 
itself in the increased complexity of the association elements. 
This takes place especially in the brain or highest centre, and 
it is in this respect especially that the brain of the crayfish 
and insect is in advance of that of the worm. 

444. The central nervous system of the Vertebrates is wholly 
dorsal and consists of the brain and spinal cord. It originates 
from the ectoderm as a tube, but the cavity of the tube remains 
extremely small, except in the region of the brain, where a series 
of chambers of considerable size are developed. The nerve 
elements develop in the walls of this tube and form a continu- 
ous ganglionic mass. The nerves are arranged segmentally, 
but there is little evidence of segmentation in the brain or spinal 
cord. In a cross section of the spinal cord the nerve cells are 

seen to be massed in the central portion in an area resembling 
a letter H. The space around this is made up of longitudinal 
nerve fibres, which are each one encased in a thick sheath of a 
fatty substance, which gives them an opaque white appearance 
when seen in mass. The central gray area contains naked 
fibres as well as cells. In the central parts of the brain there are 
also large masses of the cellular gray matter, but a still larger 
quantity of the gray is distributed over the surfaces of the folded 


FIG. 102. Brain of a shark, Notidanus. A, Dorsal view; B. lateral view. The 
roots of the nerves are represented in black. (From Johnston.) 


cerebral and cerebellar regions. Hence, the surface of the brain 
is largely .composed of the gray matter, while the fibre tracts 
are wholly beneath the surface. In the human body there are 
forty-three pairs of nerves, twelve connecting with the brain 
and thirty-one with the spinal cord. A typical spinal nerve 
is connected with the spinal cord by two "roots," one dorsal 
and one ventral. On the dorsal root, not far from the spinal 
cord, there is a ganglion, and immediately beyond the two roots 
unite to form the spinal nerve. The latter then divides into 

FIG. 103. Diagram of a cross section of the spinal cord and the roots of the 
spinal nerves. C, Central canal; df, dorsal fissure; dr, dorsal root of spinal nerve, 
arising from the dorsal horn of the gray matter (g) ; gn, ganglion on the dorsal 
root; n, spinal nerve; a/, ventral fissure; vr, ventral root of the spinal nerve, 
arising from the ventral horn of the gray maater; w, white matter. (From 
Galloway, by Folsom.) 

three main trunks; one passes into the body cavity to connect 
with the sympathetic system which supplies the viscera, one 
passes up to the muscles and skin of the dorsal portion of the 
body, and the last turns downward to the muscles and skin 
of the ventral portion of the body. Both dorsal and ventral 
branches contain both receptor and effector fibres, but at the 
juncture of dorsal and ventral roots the two classes of fibres 
separate, the effectors pass over the ventral root and the recep- 
tors pass over the dorsal root. The cells of the receptors lie 
in the spinal ganglion of the dorsal root, while the cells of the 
effectors lie in the ventral horn of the gray matter in the cord. 


445. The spinal nerves are quite uniform in their relations 
so far as described above, but the cranial nerves are consider- 
ably modified. The olfactory (I) and optic (II) nerves are not 
comparable with spinal nerves at all. The III, IV, and VI go 
to the muscles of the eye ball and contain no receptor elements. 
The VIII nerve is the auditory and is purely receptor. The 
V and VII nerves supply the skin and muscles of the face and 
lower jaw, and are mixed in function. The IX nerve supplies 
the muscles and sense organs of the tongue (taste) and pharynx, 
and is also mixed. The X nerve supplies the viscera from the 
pharynx to the liver, including larynx, lungs, oesophagus and 
stomach and heart. It is also a mixed nerve. The XI and 
XII are chiefly effector in function; they supply chiefly muscles 
of the neck region. 


446. When an animal puts itself in motion, work is being 
done, as is the case when any other body is being moved, and 
when work is being done there is an expenditure of energy. 
However, throughout its life the animal is continually moving 
itself as well as other bodies, and hence, as constantly expending 
energy. And for a considerable portion of its life its capability 
of expending energy increases, even though energy is constantly 
being spent. Now, a fundamental postulate of physics says 
that energy is never created, but that wherever it appears it 
has merely been transformed or transferred from some other 
source. The animal may be exhausted temporarily and yet 
after a while its power of expending energy is renewed. And 
we know that the condition upon which this renewal of energy 
depends is the supply of proper food to the organism. Thus 
the food is apparently the energy source for the animal. 

447. If we analyze the foods of animals we find that the most 
important by far, for their energy-yielding value, as food, are 



Available constituents. 


% Proteid. 

Fats %. 



i ~ 

Mutton .... 






2O A. 





Coif rinrk 

23-0 . 
8 i 

T ., f. 




12. 2 

_ _~~ __~ 


3 2 




T arrl 






6 A 


Flour (wheat) . . 




Wheat bread . . . 


Potatoes (Irish). 










Peas (green) .... 




I .0 


Apples. . . 




Olive oil 




Available constituents. 



Water, %. 


71 "% 


7 C O 



76 o 



60 o 



76 b 
64 o 


Salt pork 
Sausage .... 



4 x -5 




Q 7 A 

2. I 

/ -4 







L - 

Flour (wheat).. , 


" 5 


Wheat bread . . 


Potatoes (Irish) 

20. o 

3o- 7 



90 o 



OO t 


2. 2 

Q2 C 


y.5 o 

Lettuce . . 


1 1 j 



y4- o 

Q { Q 

Sugar. . . 


Olive oil 



the carbohydrates and fats. We also know that these sul 
stances may be readily made to yield energy by heating thei 
in the presence of oxygen. The carbohydrates are then decom- 
posed and their constituents unite with oxygen to form carbon- 
dioxide and water. This union produces heat, which is a form 
of energy. 

448. The wood in the firebox of an engine is a case where 
C 6 Hio05+602 = 6C02+5H 2 O. The heat liberated in this 
process is taken up by the water, which, -because of the added 
energy, expands into stem. The energy of the enclosed steam 
is evident in the pressure which it exerts, and by means of the 
mechanism of the engine, the pressure energy is transformed 
into the motion of piston, crankshaft and wheels. Thus the 
oxidation of the fuel releases the energy which causes the 
engine to move and do work. Fats and oils, C m H n (O) when 
burned yield C0 2 and H 2 and energy is also set free in the 
same way. 

449. That in these processes it is a question of liberation 
rather than the creation of energy becomes clear if we consider 
the origin of the substances. The carbohydrates, we will recall, 
were formed in the leaf from CO 2 and H 2 O, through the agency 
of chlorophyll and light. But light is a form of energy whose 
source is the sun. In some way the energy of the ether vibra- 
tions breaks up the C0 2 molecule and the carbon atom unites 
with the H 2 molecule, producing, as a final result, starch 
(Ce HIQ 65), sugar, cellulose, oil, proteid or other carbon com- 
pound. These substances remain stable at ordinary tempera- 
ture, but when slightly heated they break down, and the C ther 
unites with the and the energy which was stored up in photo- 
synthesis is again set free. In physical terms, the energy oi 
the light becomes potential energy in the chemical compound 
and again kinetic energy of heat in combustion. 

450. The processes by which the food yields energy to the 
animal are closely analogous to the case of the fuel in the 


steam engine, but it will be necessary to point out some 
differences and show how the animal engine works. 

451. In the first place, the animal "fire box" is not the 
stomach or lungs or any similar organ. The combustion takes 
place in the cells and each individual cell of the animal body is 
a unit so far as this process is concerned. Some cells require 
more fuel than others in proportion as some are greater workers 
than others. Consequently the muscle cells require much fuel. 
But how does the food get to the muscle cells? That is another 
question, and to make it clear we will return to the case of the 

452. Digestion. When the amceba comes in contact with a 
particle of food its protoplasm flows around the particle until 
it is entirely enclosed and lies embedded in the protoplasm, 
but with the particle there is also engulfed a droplet of water. 
This is called a food vacuole. The water in which the amceba 
lives is always slightly alkaline and the protoplasm of the amceba 
is also alkaline, but if delicate test is made it is found that the 
water or fluid of the food-vacuole becomes slightly acid and 
soon changes may be observed in the food particle. If it 
was a living object it soon dies; if it was a blue green 
alga, the blue color is rapidly diffused into the surround- 
ing medium. The vacuole becomes alkaline, and the food 
substance becomes translucent as though it were being 
dissolved, and gradually it disappears. Probably some por- 
tions remain unchanged. The vacuole grows smaller until the 
unchanged portions of the ingested object are closely surrounded 
by protoplasm. Finally, what remains in the food vacuole is 
ejected. While this has been going on, other food vacuoles 
have been formed and the same series of phenomena take place 
in each. All the while the animal is growing larger at the ex- 
pense, evidently, of the substance of the food- vacuoles. It is 
important to note that the food substance of the vacuoles 
disappears and later reappears as protoplasm. Between these 


two stages there is a stage when this substance is invisible in 
solution, and it is during this stage that it passes from the 
vacuole into the protoplasm. 

453. Let us recall the phenomena of fermentation as exhib- 
ited by the yeast plant or bacteria. Here we have living cells en- 
closed in a membrane, through which no visible particle is known 
to pass. The bacteria live in a watery medium surrounded 
by solid substances, upon which they are nourished. In the 
medium there appear substances which are secreted by the liv- 
ing cells and which act upon the food substances in such a way 
as to cause them to go into solution. This process may come 
about in many different ways, but the result is always a solution 
which may be absorbed by the cell through the membrane. 
The chief difference between the bacteria and the amoeba, with 
respect to the way in which the food is prepared, so that it 
may be absorbed, is this: The bacteria fill the surrounding 
medium with an enzyme which dissolves the food substances 
there. The amoeba takes into its body a droplet of the medium 
containing a particle of food, and into this droplet of the medium 
it secretes a digestive fluid. We may transpose terms and say 
that the bacteria digest the food before it is taken into the 
body and the amoeba carries on a process of fermentation 
within its food-vacuoles. That is to say, digestion is a matter 
of fermentation. 

454. The term gastro-vascular cavity applied to the central 
cavity of hydra indicates that it is analogous to the stomach, 
and hence, concerned in digestion. The food unquestionably 
passes into this cavity, but to what extent it is there digested is 
uncertain. Small particles are known to be captured by the 
flagellate cells of the entoderm and engulfed by the protoplasm. 
So that in this case as well as in Sponges and some Flat-worms 
the digestion resembles that of amoeba. In this case the func- 
tion of the gastro-vascular cavity would be to serve as a sort of 
trap for the food particles. But frequently objects are captured 



and swallowed which are much too large to be taken up by a 

cell. In the sea anemone such objects undergo partial dis- 

integration in the gastro-vascular cavity, and the fragments 

are taken up by the cells. The hydra lacks 

the organs by which this is accomplished by 

the anemone, but it is still probable that the 

close application of the walls of hydra to the 

prey may accomplish the same end. The 

undigested portions of the food are cast out 

at the mouth. 

455. The elongated form of the body of 
the worm makes possible a considerable ad- 
vance in the digestive system. The digestive 
cavity is a slender tube opening to the ex- 
terior at each end. The food is taken in at 
the mouth, and as it passes slowly along the 
narrow channel it is gradually digested and 
absorbed. The parts that remain undigested 
are cast out at the vent. The elongated 
form makes possible the successive applica- 
tion of different agencies of digestion to a 
given particle and the simultaneous opera- 

FIG. 104. The in- 

tion of these agencies in different parts of testine of a worm 

the canal. In nereis there are jaws and 

denticles by which the food is captured and ing of the glandular 

forced into the mouth, and perhaps, to some body-wall' "is*' repre- 

extent, lacerated. There is a pair of "sail- ^ ed i 5 outl if e ' 

B.C., Body cavity; 

vary" glands which open into the anterior C.E., glandular epi- 
end of the digestive tract and throughout ^ um ' } Int} l 
the remainder of its length the intestinal epi- 
thelium is thickly studded with unicellular glands, which also 
pour a secretion into the digestive cavity. In the earthworm 
the digestive canal is more differentiated. 
456. In Worms we have unquestionably a case of a true 


digestive cavity, into which the digestive enzymes are se- 
creted, in which digestion takes place and from which the 
soluble products are then absorbed. The long digestive tube 
gives a large absorbing surface, but in some cases, as in the 
earthworm, the surface is further increased by a longitudinal 
fold which hangs from the dorsal side of the canal and gives the 
lumen of the canal a crescentic form in cross section. The 

FIG. 106. 

FIG. 105. FIG. 107. 

FIG. 105. Cross section of the intestine of nereis showing the glandular 
epithelium and blood capillaries (black). 

FIG. 106. A part of the preceding figure enlarged. The upper two-thirds 
of the figure is the epithelium. Below that is a blood capillary. Then follows 
a layer of longitudinal muscle fibres cut across and a layer of circular muscle 
fibres lying in the plane of the section. The lower layer is an extremely thin 
epithelium lining the outer surface of the intestine. 

FIG. 107. A surface view of the inner surface of the intestinal epithelium. 
The cells are outlined by a network of supporting fibres. 

salivary glands are a simple type of a compound gland. The 
glandular epithelium is pushed outward into the body cavity 
and is greatly folded so that a large glandular surface occupies 
a small space. The part by which the gland is connected with 
the intestine forms a duct through which the secretion is poured 
into the digestive cavity. In nereis the greater part of digestion 
is doubtless due to the activity of the unicellular glands. 
457. The glandular intestinal epithelum is only a lining of a 



tube which is composed largely of muscle fibres. The muscles 
by peristaltic contraction force the contents of the canal slowly 
backward. They also regulate the size of the canal as the 
volume of the contents may demand. 

FIG. 1 08. Wall of the intestine in a small aquatic Annelid, Chaetogaster- 
There are only two thin layers of cells, one (a) which forms the lining of the 
body cavity and (b) the intestinal epithelium. 

458. The separation of the digestive processes advances a 
step farther in Arthropods and the digestive tract is divided 
into well-defined regions. In the crayfish there are, in the 
immediate region of the mouth, six pairs of segmental appen- 
dages which are modified for grasping and tearing up the food. 

FIG. 109. A section through one of the folds of the intestinal epithelium of 
nereis, showing a few of the glandular cells. The inner ends of the greatly 
elongated cells are filled with a granular secretion. The accumulation of the 
secreted substance in the ends of the cells causes them to swell and hence throws 
the surface into folds. 

From the mouth a short cesophagus leads into a large muscular 
stomach, which consists of two divisions. The first is lined 
with chitin and is provided with a mechanism consisting of 
several chitinous hooks or teeth and a set of muscles for oper- 
ating them. By means of this the food is still further broken 
up. A pair of large digestive glands lying in the body cavity 



communicate with the second smaller portion of the stomach 
by means of short ducts. Digestion proper takes place in this 
portion of the digestive tract. A narrow intestine of simple 
structure leads to the vent at the posterior end of the abdomen. 
459. The function of salivary glands is primarily to 
moisten the food preparatory to swallowing. Consequently 
they are only necessary in terrestrial animals. The so-called 
salivary glands of nereis owe their name to their position and 

FIG. no. Digestion and circulatory systems of the crayfish. Upper figure: 
a, Mouth; ,b oesophagus; c, cardiac portion of stomach; d-e, pyloric portion of 
stomach; e, opening of digestive gland; /, intestine; g, vent; h, digestive gland 
("liver"); i, heart; j, gonad; k, brain; /, /, ventral nerve cord. Lower figure: 
a, Heart; b, dorsal abdominal artery; c, sternal artery which branches into the 
ventral abdominal and the ventral thoracic arteries; d, ophthalmic artery; 
e, antennary artery; /, hepatic artery; g, blood sinuses; h, afferent branchial 
vessels; i, efferent branchial vessels. 

must not be supposed to be in any sense true salivary glands. 
No such glands occur in the crayfish, but in the terrestrial 
Arthropods, the Insects, they are generally found. The mouth 
parts in Insects consist of three pairs of appendages. There 
is an oesophagus, into which the salivary glands open. Some- 
times the oesophagus is enlarged to form a crop. Sometimes 
there is also a gastric mill analogous to that of the crayfish. 
Then follows the true stomach which has numerous small 


glands embedded in its thick walls or else there are larger 
glands lying outside the stomach wall, but connected with it by 
ducts. From the stomach the digestive tract continues, first 
as a slender "small intestine" which farther on expands into a 
wider " large intestine." The latter ends at the vent. 

460. Quite generally the digestive tract of Vertebrates is 
differentiated into the following series of parts; buccal cavity, 
oesophagus, stomach, small intestine, and large intestine. Its 
walls are very muscular, especially those of the stomach and 
small intestine. The internal surface area is greatly increased 
by folds and countless minute thread-like elevations, the villi. 
The teeth with which the mouth is usually armed serve either 
for seizing and swallowing the prey or for mastication. 

461. The digestive glands are numerous and large. In man 
there are three pairs of large salivary glands, besides a number 
of smaller ones, which open into the buccal cavity. But for 
Vertebrates the general statement also applies, that salivary 
glands are characteristic only of terrestrial forms. Besides 
moistening the food for swallowing it the saliva also sometimes 
seems to soften it preparatory to digestion (birds). In herbiv- 
orous animals the saliva often contains an amylolytic ferment, 
ptyalin. Embedded in the thick walls of the stomach are 
numerous small tubular glands which secrete gastric fluid. 
This contains hydrochloric acid and a proteolytic ferment, 

462. A gland of considerable size, the pancreas, opens into 
the intestine near the stomach. Its secretion contains three 
ferments, one, amylopsin, is amylolytic; another, trypsin, is 
proteolytic, and a third, steapsin, decomposes fats into glycerine 
and fatty acids. There are also numerous small glands em- 
bedded in the wall of the intestine, which are said to secrete the 
ferment, invertin, which is found in the intestine and which 
inverts maltose into glucose. 

463 . Amylolytic ferments are not all alike. That is, there are 





FIG. in. Diagram of the digestive tract of man. A.C., Ascending colon; 
C, cardiac portion of the stomach; C.B.D., common bile duct; Ca, caecum; 


a number of substances, derived from different sources, which 
have the power of changing starch into sugar, but produce this 
result under different conditions. This indicates a difference 
in constitution of the ferments. The same is true of the pro- 
teolytic ferments, for example, pepsin and trypsin are both 
proteolytic, but the one in acid media, the other in alkaline, 
and in Cephalopods there is a ferment which resembles both of 
these. Generally, in higher animals, there are more kinds of 
ferments, but each is more circumscribed in its action. Con- 
versely, in the lower forms, the ferments are fewer in kind but 
more general in action. So there is apparently in the higher 
animals a differentiation of ferments to correspond with the 
structural differentiation of the digestive tract. 

464. In the food vacuole of amoeba the medium first 
becomes acid but later, at the time when the food particles are 
disintegrating, the reaction is alkaline. Proteolytic and amy- 
lolytic ferments are present, and these seem to vary with differ- 
ent types of Protozoa and the exact nature of those found in 
amoeba is not certainly known. 

465. The fluid of the gastro-vascular cavity of Ccelenterates 
has no amylolytic powers. There is a slight proteolytic action 
which probably serves to dissociate large objects so that the 
particles may be ingested by the entodermal cells. 

466. The digestive fluids of the earthworm and nereis are 
both tryptic and diastatic. The earthworm covers leaves it 
means to swallow with saliva and allows them to digest for 
some time before swallowing them. 

467. The digestive gland of the crayfish has strong proteo- 
lytic and amylolytic action in both acid and alkaline media. 
Hence, it resembles both gastric and pancreatic digestion of 

D.C., descending colon; Duo, duodenum; Ep.Gl., epiglottis ;G.B., gall bladder; 
H.D., hepatic duct; II, ileum; OC, oral cavity; Oes, oesophagus; P, pyloric portion 
of the stomach; Pa, pancreas; P. D., pancreatic duct; Ph, pharynx; P.G., parotid 
gland; R, rectum; S, stomach; S.L., sublingual gland; S.M., sub maxillary gland; 
T, tongue; T.C., transverse colon; Tr, trachea; v.A., vermiform- appendix. 



Vertebrates. In the cockroach the salivary glands have an 
amylolytic action. The intestinal fluids have amylolytic, pro- 
teolytic and inverting action, and the reaction is neutral and 

468. Circulation. However simple or complicated the diges- 
tive processes may be, the result is essentially the same. The 
end finally attained is food substances prepared for absorption. 
This is a function entirely distinct from digestion, and since 

each cell must absorb food for itself, 
little differentiation is to be looked for 
in connection with this function. How- 
ever, only those cells can absorb which 
are in contact with the food, i. e., the 
cells lining the digestive tract. Those 
farther removed must receive their por- 
tion from those nearer the source. In 
hydra no cell is more than one cell re- 
moved from the seat of digestion, for 
the gas tro- vascular cavity extends to 
all parts of the body, even to the tips 
of the tentacles, and whether digestion 
takes place in the gastro- vascular cavity 

or in the entodermal cells the ectoderm cells are only one cell 

layer removed. 

469. In the smallest Annelids the intestinal wall is very thin. 
The same is true of the body wall, and the two are separated by 
a space, the body cavity, which is filled with a fluid ("body 
fluid"). This body fluid nourishes the tissues bathed by it 
and it is constantly replenished by the substances absorbed by 
the intestine. The movements of the animal force the body 
fluid about so that it becomes throughly mixed and freshly 
absorbed matter is thus directly brought to the farthest tissues 
of the body. 

470. In the larger worms, however, the tissues are often so 

FIG. 112. A view of the 
outer surface of the intes- 
tine of nereis, showing a 
network of blood capillaries 
and two sets of slender mus- 
cle fibres crossing each other 
at right angles. 



thick that the deeper lying cells would be starved by such a 
method of food distribution. Moreover, the wall of the digestive 
tract is so thick that it would greatly impede the transfer of 
absorbed food to the body fluid. There is, therefore, necessary 
a system of channels by which the food may more readily be 
transferred from the seat of digestion to the place of assimila- 



FIG. 113. The circulatory system of annelids. A, A longitudinal section of 
a blood-vessel of a small fresh-water annelid (Chaetogaster) showing extremely 
thin walls. B, Cross section diagram of nereis to show the arrangement of the 
vessels; D.V., dorsal vessel; Int, intestine; N. nephridium; P.V., parapodial ves- 
sels; V.I., intestinal vessels and capillaries; V.V., ventral vessel. All vessels 

tion. These channels consist of a network of tubes of extremely 
small calibre, which penetrate to every part of the intestinal 
wall, immediately outside the intestinal epithelium. Larger 
vessels lead from this network of capillaries to a much larger 
vessel which runs longitudinally along the mid-dorsal line of 
the body. In each segment branches of the dorsal vessel lead 
out laterally to the muscles, epidermis and all other organs of 


the body, where they divide into another network of capillary 
vessels, through which the blood is distributed to all the tissues. 
From this second system of capillaries larger vessels lead to 
another large, longitudinal vessel lying between the ventral 
nerve cord and the intestine. This vessel is connected with the 
intestinal system of capillaries, and thus a complete circuit is 
formed. The larger vessels are muscular, and by their rhythmi- 
cal contraction the blood is forced along. This is especially 
true of the dorsal longitudinal vessel, in which a continuous 
series of contractions pass forward from posterior to anterior, 
forcing the blood along in the same direction. In the ventral 
longitudinal vessel the blood flows from anterior to posterior. 

471. In the system just described the vessels are "closed"; 
that is, they do not open into the body cavity, and they con- 
tain blood, which is not the same as the body fluid. In many 
invertebrates, however, the body cavity forms a part of the 
system of spaces through which the blood circulates and in this 
case there is no distinction of blood and body fluid. This is 
the type of circulatory system found in Crustacea and Insects. 
A part of the dorsal vessel is much enlarged and very muscular. 
By its contraction the blood is forced forward and backward 
through branching vessels to all parts of the body. On its 
return the blood enters large spaces, which represent the body 
cavity and thus it reaches a space immediately surrounding 
the heart, the pericardial cavity. The heart is pierced by six 
or eight pairs of openings guarded by valves. When the heart 
expands the blood enters by these openings (ostia), but when it 
contracts the closing of the valves prevents the return of the 
blood through the ostia. It is therefore forced out through the 
vessels. (See Fig. no.) 

472. In Vertebrates the circulatory system is always closed 
and the heart is developed into a powerful pumping organ with 
two, three or four chambers. From the intestine the absorbed 
food is first carried to special organs in which it undergoes 



FIG. 114. Diagram to show the general plan of the circulation in mammals, 
i, Left ventricle; 2, aortic arch; 3, dorsal aorta; 4, postcaval vein; 5, right 
auricle; 6, right ventricle; 7, pulmonary artery; 8, pulmonary veins (the pul- 
monary veins open into the left auricle which in turn opens into the left ventricle). 
The order of the numbers 1-8 indicates the course taken by the blood in com- 
pleting a circuit of the systemic and pulmonary circulations. 10, the thoracic 
(lymphatic) trunk; n, precaval vein. Dig., The digestive tract; H.P.V., 
hepatic portal vein; Liv, liver; P, lung. 


further changes before it is admitted to the general circulation. 
The carbo-hydrates and peptones are collected by the hepatic 
portal vein and carried to the liver, where certain substances 
are absorbed and ultimately pass back into the intestine. This 
occurs in the case of some substances which would be deleterious 
if permitted to pass into the general circulation. Excess car- 
bo-hydrates are also stored temporarily in the liver and other 
organs in the form of glycogen. The fats are broken up into 
fatty acids and glycerine, and then, after absorption, resyn- 
thesized as fats of a different kind -in the cells of the mucous 
epithelium. They finally appear as globules in the lacteal 
capillaries of the villi and thus come into the blood through the 
thoracic duct. 

473. Fats are also stored, sometimes in large quantities, 
and represent a large reserve of energy. They are usually 
found in the connective tissues, under the skin, among the 
muscles, covering the visceral organs and elsewhere. From the 
liver the absorbed food materials get into the circulation through 
the inferior vena cava, while the lacteals pour their contents 
into the thoracic duct and thus into the left sub-clavian vein. 

474. So long as the nourishing fluids remain in the blood 
vessels they can be of no service to the tissues. But the walls 
of the capillaries are so thin that the fluid portion of the blood 
can seep through. In this way the lymph arises which is found 
in all the living tissues of the body, filling the minute spaces 
between the cells. Fresh supplies of lymph are continually 
escaping from the capillaries and the impoverished lymph drains 
off out of the lymph spaces into the lymph vessels, which finally 
empty into the thoracic duct. Thus the lymph enters the cir- 
culation agin. 


475. When the fuel is consumed in the firebox, to return to 
the analogy of the steam engine, there must be free access of 


air, specifically the oxygen of the air. Otherwise the combus- 
tion will not continue, the fire will die out, and the engine fin- 
ally come to a standstill. An animal also, when deprived of air, 
soon goes into a quiescent state, and when active, the amount 
of air required varies with the energy expended. The living 
animal is also continually evolving COi, and that, too, in pro- 
portion to the energy expended. It is evident, therefore, that 
there is combustion, or oxidation of carbon, going on in the 
organism. It is known that this process takes place in the 
tissues, i. e., in the cells, and we must, therefore, account for 
the presence of oxygen in the tissues. 

FIG. 115. Part of the body of nereis, showing the respiratory organs. The 
broad superior ligula of the dorsal ramus of each parapodium has a thin integu- 
ment and is richly supplied with blood-vessels. 

476. Amoeba and many other organisms can absorb enough 
oxygen through the general surface of the body. Even com- 
paratively large animals, because of their form and peculiarity 
of structure, can obtain enough oxygen in this way. The sea- 
anemone, for example, though comparatively large, exposes 
not only the external surface of the body and tentacles, but the 
much larger folded surface of the gastro-vascular cavity is 
exposed to the water, which is being continually renewed by 
currents passing in and out of the mouth. Even the frog, when 



quiescent, may have its demands for oxygen satisfied by ab 
sorption. through the skin. But the more compactly buil 
animals, even when not large, are provided with special organ: 
for the absorption of oxygen. The earthworm is among th< 
largest of animals destitute of such organs. But the earth worn 
is unable to absorb enough oxygen when in water and wil 
ultimately drown. Nereis possesses a pair of flat plates ii 

FIG. 116. Cross section of crawfish in the thoracic region, a, Appendage 
c, carapace; cf, part of carapace covering the gill chamber; d, digestive tract 
g, gill; h, heart; /, liver; m, m', muscles; n.c., nerve cord; p.s., pericardial sinus 
r, gonad; st, sternal artery; va, ventral artery; vs, ventral blood sinus. (Fron 
Galloway, after Lang.) 

each segment, one on each parapodium, which are richly sup 
plied with capillaries lying very near the surface. These supple 
ment the general body surface in the absorption of oxygen 
Even here the worm feels the necessity of keeping the water ir 
motion in order to bring in fresh supplies of oxygen. Wher 
the animal is at rest the body keeps up a rhythmical undulating 
movement by which the water is kept in motion. 



477. The crayfish bears under a fold of the carapace, on either 
side, a large number of brush-like gills, composed essentially of 
numerous slender thin-walled filaments, through which the 
blood constantly circulates. There is also a special structure 
in the form of a curved paddle or spoon, which by its motion 
keeps the water constantly moving through the gill chamber. 
This highly efficient set of organs evidently 

makes good the deficiency in absorbing power 
of the body surface resulting from the imper- 
vious cuticular integument. 

478. In Insects, a unique method of aerating 
the body has developed. The air is carried to 
all parts of the body by an intricate system of 
slender tubes, tracheae, which open on the sur- 
face through small pores in the integument, the 
stigmata. The air is forced into and out of 
these tubes by a telescoping action of the rings 
of the abdomen. 

479. In Fishes, the gills are not unlike those 
of the crayfish, but the water current is produced 
in a different way. The water is first taken into 
the pharynx through the mouth, and from the 
pharynx it passes through a series of slits be- 
tween the arches which bear the gills. A pair of delicate 
membranes at the mouth serve as valves and cause a flow of 
water, always in the same direction, to result from merely 
opening and closing the mouth. 

480. In a few fishes and in the adult stage of all other Verte- 
brates, a pair of air sacks or lungs take the place of the gills of 
the fish. In the lower forms the lungs are comparatively 
simple, the inner surface of the air sacks being, at most, some- 
what folded. In the birds and mammals, however, they 
become exceedingly complex, through the folding of the walls to 
increase the absorbing surface. The lung first appears as a 

FIG. 117. Dia- 
gram of a feather- 
like gill. This 
type is found in 
the crayfish. 



pocket in the ventral wall of the digestive tract in the region 
of the pharynx. This pocket divides into two branches which 
develop into the right and left lungs. Each branch divides 
many times so that a very complicated system of tubes is 
formed. The air tubes are thin walled and a network of blood 
capillaries closely surrounds them. 

481. Inspiration in the Amphibia and a few Reptiles is a 
process analogous to swallowing. But in most Reptiles and in 


FIG. 118. Three early stages in the development of a mammalian lung. In 
B the alimentary canal is shown extending upward directly above the letter B. 
Ep, I, and II, the bronchial tubes. Ap, pulmonary artery; Vp, pulmonary 
veins. (McMurrich, after His.) 

Birds and Mammals the air is forced into the lungs by atmos- 
pheric pressure upon muscular expansion of the thoracic cavity. 
The latter is brought about by elevation of the ribs and, in 
Mammals, by depression of the dome-shaped diaphragm. 

482. The course which the blood takes may or may not have 
a fixed relation to the respiratory organs. In Worms and 
Crustacea some blood is continually being oxygenated, and this, 
mingling with the rest, is sufficient for the needs of the animal. 
In Fishes, all the blood passing through the heart is forced 
through the gills and then passes on to the tissues of the body. 


In Frogs and Reptiles the oxygenated blood coming from the 
lungs and that coming from the other tissues of the body mingle 
to some extent in the heart, and this mixed blood is then sup- 
plied to the tissues of the body. In Birds and Mammals again, 
through the complete separation of the respiratory and systemic 
circulation, all blood passes alternately through the lungs and 
the body tissues. 

483. Oxygen is taken up by the blood as air is absorbed by 
water, but in most animals, excepting Insects, there is a sub- 
stance present in the blood which has a special affinity for oxy- 
gen. In some cases, especially among invertebrates, this 
substance forms part of the blood plasma; in others, including 
all Vertebrates, it resides in certain cells floating in the blood, 
the red blood corpuscles. In either case it gives the character- 
istic color to the blood. In some invertebrates, the earthworm, 
for example, and all Vertebrates, the substance is red and con- 
tains iron. In other cases, some Crustacea and some Mollusca, 
the substance is blue and contains copper. The first is called 
haemoglobin, the latter haemocyanin. There are also some 
others, more rare. These substances have an affinity for 
oxygen, so that the blood is enabled to carry more oxygen than 
it otherwise could. In passing through the respiratory organs 
the oxygen carriers become charged with oxygen and assume a 
brighter color. In passing through the tissues where oxygen is 
needed the haemoglobin, or haemocyanin, again assume a darker 
color because of the loss of oxygen to the tissues. The red 
corpuscles originate in the red marrow of the bones. They are 
short lived and disintegrate in the liver and form the red and 
green pigments of the bile. 


484. In green plants, the protoplasm takes up inorganic 
substances, such as water, carbon-dioxide, nitrates and other 
mineral salts containing sulphur, phosphorus, iron, calcium, 


magnesium, potassium, and others. From these the simpler 
organic compounds, such as the carbohydrates, are formed, also 
more complex nitrogenous substances like aleurone and finally 
protoplasm itself. Animals, however, lack the power of build- 
ing up protoplasm from its inorganic constituents. They 
require food containing organic nitrogenous compounds like 
aleurone, albumen and protoplasm. These may be supple- 
mented by the simpler carbon compounds, like the carbo- 
hydrates, fats and oils. The nitrogenous substances, are 
necessary wherever growth or repair are taking place, i. e., 
wherever protoplasm is being formed. The carbon compounds 
may be used as well as the nitrogenous where there is merely an 
evolution of energy demanded, as in locomotion and the produc- 
tion of heat. The details of the processes which take place in 
the cell are not known. But when foods are assimilated, 
growth takes place and the cell becomes energized so that it is 
capable of performing the functions peculiar to it. 

485. The results of the activities of the cell may be briefly 

486. Growth is the most general result of assimilation, but 
need not be further discussed here. 

487. In glandular cells, activity results in the formation of 
the special secretions which are characteristic of the gland. 
These may be used in the building up of permanent structures 
of the organism, such as bone or cartilage, or the secretions 
may have only a temporary value, and after they have served 
their purpose, be eliminated from the body as slime and oil 
from the glands of the skin. With this class may be included 
those cells which produce substances by the transformation of 
protoplasm, although in the true glandular cell the secretions 
have probably not reached the complexity of structure of proto- 
plasm. Cuticular and epidermal structures are of the trans- 
formed protoplasm type. 

488. The activity of muscle is manifested primarily by a 



contraction and secondarily by the production of heat, but at 
the same time, substances are formed and set free, which show 
that chemical processes are at work and which give a clue as to 
the nature of those processes. The oxygen and food substances, 
C 6 Hi 2 O 6 , let's say, make their appearance again, but in an 
altered form. Carbon dioxide is produced in large quantities 
together with other substances which must be eliminated from 
the body, or else serious disorders occur. Nitrogen waste 
compounds are also formed, and like the CO2, they have a lower 
energy value than the substances from which they were derived. 


489. The waste matters produced by metabolism are soluble 
in the body fluids and in water. Hence, small animals like 
amoeba and hydra can eliminate them from the body surface by 


FIG. 119. Diagram of a nephridium of an Annelid, b, b f , blood-vessels; 
c, coelom; d, duct; e, opening through the epidermis; /, funnel; gl, glandular 
portion; s, mesentery; W, wall of body; iv, wall of intestine. (From Galloway.) 

diffusion. In larger animals the substances excreted by the 
cells pass out into the body fluid or the blood current and are 
thus carried to the place of elimination. The CO2 being a gas 
is chiefly given off from the organs of respiration, following the 
path of the oxygen, but in the reverse direction. The volume of 
oxygen absorbed in the human lung'is about 5 per cent, of the 
inspired air. The volume of C0 2 given off is a little over 4 


per cent, on the average. If only carbohydrate foods were 
assimilated the percentage of these gases should be equal, but 
the oxygen consumed with hydrocarbon and proteid foods in 
part leaves the body by another path. 

490. The nitrogenous waste matters are not gaseous and, 
therefore, cannot be eliminated by the lungs, and in fact, we 
find in all the higher animals a special set of organs for this 
function. The organs which presumably perform this function 

FIG. i2o.-^A section through the nephridium of nereis showing the funnel in 
longitudinal section and the convoluted tubule cut across at many points. The 
blood-vessels are also cut at several points (black). B.W., The body wall; 
B.V., blood-vessel; c, coiled tubule; F, funnel. 

in Worms are pairs of tubules arranged segmentally, one pair 
in each segment. They are called nephridia and consist of 
slender, more or less coiled, tubes which open into the body 
cavity by a ciliated funnel-like opening. The other end of the 
tubule opens by a pore on the surface of the body. 

491. In the crayfish there are organs, the " green glands," 
which are probably homologous to nephridia, but there is only 
a single pair, located at the base of the antennae. They are 
comparatively large organs and more complicated in structure. 

492. The single pair of kidneys of Vertebrates are much 
more complicated excretory organs and yet the uriniferous 
tubules of the kidney resemble the nephridia of the worm and 
are probably homologous organs. The kidney tubule is a long, 


slender convoluted tube which in the primitive condition has a 
funnel like the nephridium, but in the mature condition of the 
mammal it is closed. Instead, however, a considerable part of 
its wall is closely applied to complex knots and networks of 
blood capillaries from which the secreting cells of the tubule 
extract the nitrogen waste matter. The kidney tubules all 
open into a common chamber from which a duct, the ureter, 
leads to a reservoir, the urinary bladder. The nitrogenous 
wastes leave the tissues with the lymph and thus are carried 
back into the general circulation. In this way they reach the 
kidneys. The chief waste drawn from the blood by the kidneys 
is urea, CON2H4, but there are a number of other substances 
eliminated in much smaller volume. 

493. The liver of the higher animals seems to have several 
functions, one of which is excretion. The bile secreted by the 
liver is a complex substance and its significance is not fully 
understood. Its function in digestion is probably only a 
secondary one. It contains waste matters taken from the blood 
and these are eliminated through the intestine. 

494. The white blood corpuscles also assist in ridding the 
body of useless or deleterious substances. 


495. Under favorable conditions an amoeba will occasion- 
ally divide into two similar parts. These parts then continue 
to grow and after a time they also divide. This phenomenon 
is one of the characters of the living cell. The impulse to divide 
does not seem to depend upon any special external stimulus. 
It is the normal consequence of growth under favorable condi- 
tions. The process of division requires from a few minutes to 
an hour from beginning to completion, and may be repeated 
after a number of hours to several days. 

496. With regard to the details of the process of division there 
are two types. In one case it is much more complicated than 


in the other. In the simpler type the first evidence that di- 
vision is about to take place is seen in a slight elongation of the 
nucleus. This proceeds until the nucleus assumes the shape of 
a dumb-bell. The two halves continue to draw apart until 
only a slender strand connects them and this finally breaks. 
As the division of the nucleus proceeds the body of the cell also 
elongates. The pseudopodia are formed only at the two ends. 
The cell becomes constricted in the equatorial plane and this 
cuts deeper into the cell until the latter is finally cut into 
two approximately equal parts. The two daughter nuclei have 
by this time assumed the normal rounded form and there are 
then two amoebae. In the division the contractile vacuole 
remains in one of the daughter cells, but before division is 
complete a new vacuole has been formed in the other one. 

497. In other cases, division comes about through a compli- 
cated process known as mitosis or karyokinesis. This process 
is described below. No significance is known to attach to the 
difference in method. The results are apparently the same. 

498. In many Protozoa, another interesting phenomenon 
has been observed which should be mentioned here, although 
it has not been observed in amoeba. This is the phenomenon 
of conjugation. Two similar animals unite, either partially 
and temporarily or else completely, so as to form a single cell. 
In the latter case the two nuclei fuse into one. When the 
union is only temporary the nuclei of both cells divide and a part 
of the nuclear matter from each cell is transferred to the other 
cell, where it unites with the nucleus of that cell. By either of 
these processes cells are formed with nuclei composed of ma- 
terial derived in equal parts from two individuals. The 
details of this process will be discussed more fully in Part III. 
Its significance will be better understood when compared with 
the sexual method of reproduction of the metazoa. 

499. Hydra reproduces by budding and by development of 
eggs. Budding is a process found among other metazoa as 



well as among the Coelenterata. The process is well exempli- 
fied by hydra. The bud which is eventually to form a new 
polyp is first seen as a slight protuberance of the lower part of 

FIG. 121. Diagram of hydra in longitudinal section. A, Well-developed bud 
is shown on the right; B, base; o, ovary; T, testis. (From Korschelt and Heider, 
after Aders.) 

the wall of the column. This is caused by the more rapid 
growth of the tissues in this region and involves both ectoderm 
and entoderm. The bud grows larger, becomes cylindrical, 
and finally a circle of tentacles forms around the distal end. 



The bud resembles the parent polyp in form and may be half 
as large. With the opening of a mouth in the centre of the 
circle of tentacles the animal is complete. Up to this time 
the gastro- vascular cavity of the bud has been in communica- 
tion with that of the parent, but the base of the bud becomes 
gradually more constricted until finally the bud is cut off 
entirely and is then an independent organism. Several such 
buds may be in process of development at one time and by this 
means the number of individuals rapidly grows. 

FIG. 122. The egg cell of hydra, in amoeboid 
form. (After Kleinenberg.) 

FIG. 123. A hydra embryo. 
The first four tentacles just 
beginning to develop. (After 

500. Less frequently another type of protuberance may be 
observed on the column of the hydra. Just below the circle of 
tentacles may be found a conical eminence which affects only 
the ectoderm. Lower down on the column, frequently on the 
same individual, a somewhat similar, though more rounded, 
protuberance may be found. These are the gonads. The 
upper ones are testes and in them are developed the sperm cells, 
which are very small and provided with a flagellum. These are 
produced in large numbers. The lower gonads are the ovaries 
and contain finally a single large cell, the ovum. Both ova 
and sperm cells are derived from the ectoderm, but they recede 
from the surface and are covered by the ectodermal epithelium 
during the period of development. When the egg is mature it 



becomes exposed by the breaking of the ectoderm, but it still 
remains attached by a stalk. At this time the sperm cells are 
liberated from the testis in large numbers. They swim about 
in the water and by some means, probably a chemical stimulus 
originating in the egg, they are attracted to the egg. One of 
the sperm cells penetrates the protoplasm and fuses with the egg 
nucleus. This "fertilizing" process initiates the developing 
process. A membrane is first formed around the egg and by 
repeated cell division a cylindrical embryo is developed. The 
membrane then breaks and the ciliated larva is set free at the 

FIG. 124. Longitudinal section of small Turbellarian, Microstomum, which 
multiplies asexually by strobilation. b, Brain; c, ciliated pit; d, planes of 
division; e, eye-spot; ent, entoderm; g, intestine; gl, gland cells; m, mouth 
(original); m f , mouth of second zooid; m 2 , m 3 , mouths of offspring of second and 
third orders. The strobila consists of a chain of four nearly completed zooids. 
(From Galloway). 

time when four tentacles are just beginning to develop. After 
swimming for a time the larva becomes attached and a mouth 
is formed. From three to five more tentacles appear in the 
spaces between the others and the young hydra is complete. 
After maturing a number of ova the parent hydra dies. 

501. Some annelid worms also reproduce by asexual methods, 
but among the higher forms like nereis and the earthworm repro- 
duction is wholly by the sexual method. In nereis the sexes 
are distinct; each individual produces either eggs or sperm, but 
not both. The reproductive cells are differentiated in size and 
form, very much as in hydra. They are developed from cells 
of the mesodermal epithelium lining the body cavity (on the 


wall of the digestive tube). When they are mature they lie 
free in the body cavity, from which they escape into the water 
by the breaking of the body of the worm. The sperm and ova 
escape into the water at the same time. After fertilization a 
larva is developed which has no resemblance to an Annelid 
but is much more like a rotifer. This is called a trochophore 
larva and is regarded as indicating relationship between the 
Rotifers and Annelids. From one end of the trochophore larva 

FIG. 125. The ovum of nereis. Photomicrograph; greatly magnified. 

the body of the worm is developed segment after segment. 
The development of nereis is, therefore, by metamorphosis. 

502. The earthworm is hermaphroditic, i. e., both sexes are 
united in one individual, and development is direct. 

503. In the crayfish the reproductive cells are developed in 
special sack-like organs lying in the thoracic part of the body 
cavity. The sexes are separate. There is a single ovary, 
consisting of a pair of lateral lobes connected by a single median 
lobe. A pair of ducts lead from the ovary to the basal joints 
of the eleventh appendages where they open to the exterior. 
The testis of the male is similar in position and composition, 


but the ducts open at the bases of the appendages of the thir- 
teenth segment. 

504. The eggs are fertilized at the moment of their escape 
from the oviducts and are then cemented to the hairs of the ab- 
dominal appendages of the female. In this way they are pro- 
tected from other animals; care is taken that they have the 
necessary supply of aerated water and they are not carried 
away by currents of water toward the sea. Even after the young 
are hatched they continue to cling for some time to the appen- 
dages of the female. 

505. It happens that development is direct in the case of 
the crayfish, though in many Crustacea there is a well marked 
metamorphosis. In some Insects development is also direct, 
as, e. g., in the grasshopper, but in several orders of Insects 
there is a complete metamorphosis. From the egg of the butter- 
fly is hatched a small caterpillar. This grows into a large cater- 
pillar. Then a metamorphosis occurs. The caterpillar be- 
comes a quiescent pupa and remains such for a time; then an- 
other change gives birth to the imago butterfly. 

506. A few fishes are hermaphroditic. In all other Verte- 
brates the sexes are distinct. The gonads are developed from 
the mesoderm. They have no ducts primarily, but certain 
tubules which belong primarily to the excretory system become 
specially modified and assume the function of genital ducts. 
In some aquatic Vertebrates the eggs are fertilized in the water, 
but in all reptiles, birds and mammals fertilization takes place 
in the oviduct and development begins before the escape of the 
egg from the body of the parent. In the highest Mammals 
this intra-uterine development continues for weeks, months or 
even, in the case of the elephant, to nearly two years. In some 
fishes, and especially in frogs and toads, there is a marked meta- 
morphosis, but in the higher groups the development is direct. 



507. PHYLUM I. Protozoa. Protozoa are found in nature 
practically everywhere where there is moisture; in the soil, in 
fresh and salt waters and even, as parasites, in the tissues of 
higher animals and plants. They may even be found in prac- 
tically dry situations, as in dust, but then only in a resting or 
spore condition. When Protozoa in this state are moistened 
they absorb water, the protoplasm swells, the enclosing mem- 
brane is broken and the organism resumes an active existence. 
This is why they always appear when a little dry soil, a few dry 
leaves or any other organic matter is placed in a dish of water. 
Many species are found the whole world over, others are more 
limited in distribution. For example, some of the parasitic 
forms are limited to one, or a few related species of host and 
consequently are limited to the range of the host. Some groups 
are peculiar to fresh waters while others are marine. 

508. The number of species of Protozoa is very great and 
there is great diversity in size, form and habits. Many are 
easily visible to the unaided eye. Many others approach the 
limit of visibility but most can only be seen with the aid of the 
microscope. The phylum is very difficult to classify but most 
forms can readily be placed in one or the other of the following 
five classes, viz., Rhizopoda, Mastigophora, Sporozoa, Ciliata 
and Suctoria. 

509. Class I. Rhizopoda. This class is characterized by 
the temporary root-like processes of the naked protoplasmic 
body, by which locomotion is effected and food ingested. A 
common example is amoeba. 




510. In the order Am&bina, to which amoeba belongs, there is no fixed 
form of body; there is no membrane, shell or skeleton of any kind. Repre- 
sentatives of this order are found in both fresh and salt waters and many 
are parasitic. Entamoeba coli is a common harmless parasite in the 
human intestine and Entamoeba histolytica is the cause of tropical 
dysentery. The order Heliozoa comprises rhizopods which have a spher- 
ical central body from which radiate numerous long, slender, ray-like 
pseudopodia. The body may be either naked or surrounded by a gel- 
atinous or silicious capsule perforated by numerous pores through which 
the pseudopodia project. Sometimes the cell is attached to other objects 

FIG. 126. Actinomma, a Radiolarian. A, Whole animal with a portion of 
two shells removed to show the interior. B, section, showing concentric shells, 
radial spines and central capsule (c) ; n, nucleus; p, protoplasm. (From Galloway, 
after Parker and Haswell.) 

by a slender stalk. Heliozoa are found in fresh and salt water. They are 
never parasitic. The order Foramenifera includes fresh- and salt-water 
rhizopods which have a shell composed of gelatinous or horny matter, to 
which may be added calcareous or silicious secretions deposited by the 
protoplasm, or minute foreign particles like grains of sand. The pseudo- 
podia may be amceboid in form or long and slender like those of the heliozoa 
but differing from the latter in the less regular arrangement and constantly 
changing form. Many species add successively larger chambers to the 
first shell. These are often in a spiral arrangement. In the order 
Radiolaria there is found a peculiar structure called the central capsule 
which encloses the nucleus and the central part of the protoplasm. Out- 


side the capsule is another layer of protoplasm which contains large 
quantities of a gelatinous secretion. Besides the central capsule there is a 
skeleton, usually of silica, composed of radial spines and concentric shells. 
This skeleton is often of a very intricate and beautiful design. The pseu- 
dopodia are slender and branching and are sometimes supported by a 
slender axial filament. The Radiolaria are marine. 

511. Class II. Mastigophora. The Mastigophora are dis- 
tinguished by the flagellum, a whip-like vibratory appendage 
by which locomotion is effected. There may be two or four or 
even a circlet of these flagella but more often there is only one. 
Flagella also occur in other groups of animals and also in plants 
but only as temporary structures. In the Mastigophora they 
are always present during the active life period of the organism. 
The body has usually a definite form though it is often capable 
of great contortion. There is a nucleus and a contractile 
vacuole. The class may be divided into three sub-classes, Flag- 
ellata, Dinoflagellata and Cystoflagellata. 

512. The Flagellata are widely distributed and there is extreme diversity 
of form and habit so that the group is difficult to characterize and classify. 
Many contain chlorophyll and are holophytic, some contain chlorophyll 
but also ingest particles of food. Some are holozoic, some saprozoic and 
many parasitic. Special examples of the latter are described in Part III. 
Those forms which ingest solid food may do so either through a definite 
oral]opening or the food may be engulfed at the surface where no preformed 
mouth occurs. In one group, the Choanoflagellata, the base of the single 
flagellum is surrounded by a collar-like membrane. The flagellum jerks 
the food particles against the outside of the collar and from there they pass 
into the cell-body. Undigested fragments of food substances are ejected 
at the base of the flagellum, within the collar. Some of the Flagellates 
form swimming colonies by the adhesion of a group of cells to each other. 
Others are stalked and adhere in groups to form fixed colonies The 
Dinoflagellata are highly specialized fresh-water or marine Mastigophora. 
The cell has two flagella, usually placed at right angles to each other, and 
hidden in deep grooves on the surface of the cell wall. The cell often has 
a very odd form and is covered with cellulose plates which are fancifully 
ornamented. Many species contain chromatophores of yellowish, brown- 
ish or greenish color. The Cystoflagellata are another small group of 



Mastigophora. They have one flagellum and locomotion may be assisted 
by rhythmical motions of the protoplasm. They are large and contain 
considerable gelatinous matter enclosed within the strong pellicula. One 
species, Noctiluca miliaris, which is found in all seas is largely responsible 
for the phosphorescence of the water. 

FIG. 127. Eimeria Schubergi, a sporozoan parasitic in the intestinal epithe- 
lium of Lithobius. A-C, Three steps in the formation of sporozoites (asexual); 
D, microgametes; E, macrogamete; F-G, fertilization; H-K, three steps in the 
formation of spores (sexual). 

51-3. Class III. Sporozoa. The class Sporozoa includes 
those protozoa which at one time in their life cycle multiply 
by the formation of spores. The spores are usually enclosed in 


a spore case but this may be wanting, as, e. g., when there is 
an alternation of hosts. The number of spores in a case is 
usually numerous but sometimes they are few or only one. All 
Sporozoa are parasitic. They are very commonly cell parasites, 
either in the young stages or permanently. They absorb fluid 
food by osmose. The Sporozoa are widely distributed and 
infect all groups of the higher animals, especially worms, arthro- 
pods, tunica tes, molluscs and vertebrates. Most species are 
limited to one or a few host species. The passage from one 
host to another similar (not alternate) host is effected in the 
spore stage. There is frequently another method of repro- 
duction which takes place wholly within a single host. This 
may alternate with the spore-producing generation thus giving 
rise to a regular alternation of generations. Many Sporozoa 
are comparatively harmless parasites but among them are also 
some of the most dangerous. Several examples are described 
under the head of parasitism. 

514. Class IV. Ciliata. The Ciliata all possess as loco- 
motor organs, numerous minute vibratile processes called cilia. 
They are widely distributed. Very few are parasitic, some are 
saprozoic, but most are holozoic. A few even are carnivorous. 
They are generally free-swimming but some attach themselves 
temporarily to other objects and some are permanently fixed 
by a stalk. The cilia serve for driving currents of water 
containing food particles to the mouth and in the fixed forms 
this is the chief function of the cilia. The form of the body is 
definite but the animal often has the power of considerably 
changing the form by contraction. The cell is covered by a 
dense protoplasmic layer called a pellicula. There are usually 
two nuclei, a large macronucleus and a small micronucleus. 
Multiplication is effected by division or by budding. Some- 
times this is accompanied by the formation of a protecting mem- 
brane or cyst. Cysts are also formed when conditions are un- 
favorable and represent a resistant condition. 


515. The Ciliata are classified on the basis of the arrangement of the 
cilia. The Holotricha have no special zone of cilia in the region of the 
mouth. The Heterotricha have a left-hand spiral of larger cilia around the 
mouth. The Oligotricha have a spiral or circle of cilia around the peri- 
stome which is anterior and at right angles to the axis of the body. Else- 
where the body is almost or wholly destitute of cilia. The Eypotricha are 
flattened dorso-ventrally. The adoral spiral is on the ventral side and the 
dorsal side is without motile cilia. The Peritricha have the adoral spiral 
right-handed, otherwise the body is not ciliated, many are stalked and 

FIG. 128. Paramcecium. A, Anterior; c, cilia; e.c., ectoplasm; e.n., endo- 
plasm; f.v., food vacuole; g, gullet; N, macronucleus; n, micro nucleus; o, oral 
groove; p.v., contractile vacuole; tr, trichocysts; v, food vacuole. 

516. Class V. Suctoria. In this group there are no organs 
of locomotion in the adult and consequently all are sessile or 
at least motionless. They are provided with long tubular pro- 
cesses by which they catch their prey. Through these tubes 
they then suck the protoplasm of the small animals they have 
caught. The young are formed by budding. They are 
provided with cilia by which they swim about for a time before 
becoming attached. Some suctoria are parasitic. The group 
is not large and is comparatively unimportant. 


517. The Metazoa are multicellular animals. In the embryo the cells 
are arranged in three distinct layers, an outer ectoderm, an kiner entoderm 



and a middle mesoderm. In the coelenterates the mesoderm is only 
incompletely developed and in some cases entirely wanting. Reproduc- 
tion is generally of the sexual type though in the lower phyla asexual 
methods often occur in addition to the sexual. 

518. PHYLUM II. Calenterata. In the Coelenterates the 
mesoderm is represented usually by a gelatinous matrix con- 
taining various cellular elements which are derived either from 
the ectoderm or entoderm. There is no body cavity or vas- 
cular system. The gastric cavity is extended by canal-like 
prolongations into all parts of the body. 

FIG. 129. Diagram of a sponge, c, Cloaca; ch, flagellale chambers; sp, in- 
current pores; ip, excurrent pores; mes, mesenchyme; o, osculum; r,c., radial 
canals. (From Galloway.) 

519. Class I. Porifera. Sponges are all marine with the 
exception of a single fresh-water genus, Spongilla. They are 
always attached to some object and sometimes bore into shells 
or calcareous rocks. They vary greatly in form; sometimes 
covering the substratum like a thin velvety crust, sometimes 
rising into conical, spherical, cylindrical or vase-shaped masses. 


They are often branched, especially the cylindrical ones and 
the more massive forms may become very irregular in shape 
through the development of new parts by irregular budding. 
Some are very delicate and fragile while others are very firm, 
even stony. The color is as variable as the form; they are often 

FIG. 130. A large cup-shaped sponge (Poterion?) from the Philippine Islands. 

X 1/8. 

a dull gray but highly colored species are very common. Or- 
ange, sulphur yellow, violet, purple and green sponges often 
give color variety to the sea bottom. The fresh- water Spongilla 
is usually green; the color in this case being due to the presence 



of minute algae imbedded in the tissue of the sponge in a sym- 
biotic relationship. In size sponges vary from a fraction of an 
inch to several feet in diameter. 

FIG. 131. Stylo tella heliophila, a typical sponge. Beaufort Harbor, N. C. 

X 1/2. 

520. A sponge is essentially a tubular structure, the walls of 
which consist of three layers, an outer thin epithelium, the 
ectoderm, an inner epithelium of collared flagellate cells, the 


entoderm, and a middle gelatinous connective- tissue matrix 
in which are embedded branched connective-tissue cells, 
calcareous or silicious spicules, primitive muscle cells and repro- 
ductive cells. Through the walls of the sponge numerous fine 
pores or slender canals penetrate from the exterior to the 
central larger canal or cloaca. In the simpler sponges the 

FIG. 132. A Niaxon sponge (Pheronema?). Philippine Islands. X 1/2. 

surface of the cloaca is lined with the collared flagellate cells. 
These have a marked resemblance to the protozoan Choano- 
flagellata and are not found in any other metazoa. The 
lashing of the flagella creates a current in the water which flows 
inward at the pores and outward at the osculum, the large open- 
ing of the cloaca. The flagella and collars together serve for 


the capture of food particles as in the Choanoflagellata and 
digestion is likewise intra-cellular. 

521. In more complex sponges the flagellate epithelium is 
limited to certain depressions of the cloacal surface which form 
chambers radiating from the cloacal cavity. These are called 
flagellate chambers or radial canals. In many cases the 
flagellate chambers are so far removed from the cloaca that 
another system of canals results, the excurrent canals, which 
connects the flagellate chambers with the cloaca. In still 
more complex forms the incurrent and excurrent canals are 

522. Sponges have no power of locomotion and only in some 
cases can any evidence of contraction be observed directly. 
However, the minute pores can be closed by the contraction of 
the muscle cells of the mesoglea. Some sponges are quite soft, 
almost jelly like, but usually the mesoglea is so filled with cal- 
careous or silicious spicules as to render the sponge firm or even 
hard. In some sponges the mesoglea contains a skeletal struc- 
ture composed of horny fibres, in addition to the spicules. 
This is notably the case with the common bath sponge in which 
the spicules are not well developed. 

523. Sponges reproduce by a process similar to budding. 
A fragment of the sponge separates, is carried away by water 
currents, becomes attached and develops into a new sponge. 
The sexual method is, however, the more frequent. The eggs 
and sperm are developed in the mesoglea, where the egg be- 
comes fertilized and begins its development. It escapes as a 
ciliated larva, swims away and becomes attached with the 
gastrula mouth down. The gastrula cavity becomes the 
cloaca and an osculum is formed by thinning of the wall at the 
end opposite the point of attachment. The incurrent pores 
are formed in like manner. 

Order i. The Calcispongiae have calcareous spicules of one, three or 
four rays. Grantia. 


Order 2. The Triaxonia are sponges with large flagellate chambers, a 
thin mesenchyme layer and triaxial silicious spicules. The latter may be 
replaced by horny fibres or, occasionally, skeletal structures are wanting. 

Order 3. The Tetraxonia have a complicated system of canals, small 
flagellate chambers and a thick mesenchyme layer. The skeleton con- 
sists of tetraxial or monaxial silicious spicules sometimes combined with, 
or replaced by, spongin fibres. Euspongia is the commercial sponge and 
Spongilla the fresh water sponge. 

524. The Cnidaria. A great many Ccelenterates are characterized by 
the possession of peculiar organs, the cnidoblasts or nettling cells. These 
occur in both ectoderm and entoderm but are often aggregated in certain 
regions, as on the tentacles. The nettling cell contains a small capsule 
which is filled with a fluid and contains a spirally wound thread. A sen- 
sory point projects at the surface. When this is stimulated the capsule 
bursts, the nettling thread is turned inside out and with it the fluid content 
of the capsule is also ejected. The effect of this discharge is to paralyze 
or kill the prey. Even the human skin is strongly irritated by the nettling 
discharge of the larger Cnidaria and from this fact arose the name. The 
Cnidaria are the Hydrozoa, Scyphozoa and Anthozoa. 

525. Class II. Hydrozoa. This class is named after the 
genus Hydra which is found in fresh waters very widely dis- 
tributed. Practically all other Hydrozoa are marine. The 
individual animals of this class are always small but many 
species are colonial and the colony may attain to considerable 
dimensions. There is usually a remarkable alternation of gen- 
erations in which an asexual, fixed polyp form alternates with a 
sexual free-swimming medusa. The polyp is in most essential 
features like the hydra in form but the lower part of the column 
is much elongated and slender thus forming a stalk. When 
budding occurs the buds are not set free but remain attached 
to the parent stem and thus is formed a colony. At certain 
times another type of bud is formed. It differs in form from 
the parent polyp and is usually set free. This is the medusa. 
Its principal axis is shorter than the radial axes so that it 
assumes the form of a saucer or bell with a fringe of tentacles 
around the edge. Near the margin of the bell on its concave 


surface a thin fold of the ectoderm projects inward toward the 
principal axis, forming a circular shelf. This is the velum, 
which serves to distinguish the medusae of this group from the 

526. From the gastric cavity four radial canals run out to 
the margin of the bell and there join a canal which runs cir- 
cularly along its edge. In some cases there are six, eight or 
more radial canals. The gonads are developed from the ecto- 
derm somewhere along the course of the radial canals or on the 
manubrium. Special sense organs, eye spots or statocysts, may 
occur along the edge of the bell and there is also a ring of nerve 
fibres. The animal has a feeble power of locomotion, effected 
by a rhythmical contraction of the edge of the bell. The hydro- 
medusae vary in size from a small fraction of an inch to two 
inches. In one group they are somewhat larger. Although so 
different in appearance the medusae are in reality of essentially 
the same structure as the polyps. They are a little more highly 
developed in accordance with the free life habit. 

527. When the eggs are fertilized a free-swimming ciliated 
larva is produced. This becomes attached by the aboral pole, 
develops tentacles and thus forms a polyp and by budding of 
the polyp a colony is developed. There is frequently more 
than one kind of polyp found in a colony. In this case there 
is a division of function so that there may be feeding polyps 
which are of the typical form; protective polyps, without ten- 
tacles and mouth but well supplied with nettling cells; repro- 
ductive polyps which produce the medusa buds for the colony. 
Sometimes the medusae remain attached to the parent colony 
and there mature the reproductive cells. In this case the me- 
dusa is more or less rudimentary. Such medusae are then 
another type of polyp and the alternation of generations 
resolves itself into a special case of polymorphism. 

Order i. The Hydroidea comprise solitary polyp forms which have no 
medusa stage, like Hydra; the Trachymedusae which have no polyp stage; 


the colonial millepore corals; and the colonial campanularian and tubu- 
larian hydroids. The colonial forms are all fixed and usually have a free 
medusa stage with alternation of generations. 

Order 2. The Siphonophora are swimming colonies in which there is a 
highly developed stock polymorphism in which certain individuals form 
floats or swimming bells. The sexual generation is either a free medusa 
or an attached medusoid bud. Physalia, the "Portuguese Man of War," 
is a well-known example. 

FIG. 133. A hydroid colony. X i. 

528. Class HI. Scyphozoa. The Scyphozoa are the com- 
mon jellyfishes. They are usually larger than the medusae 
of the Hydrozoa, varying from four inches to a yard in diameter. 
The bell is usually strongly convex and is made up of four anti- 
meres though most of the organs occur in multiples of four. 
The margin of the bell is lobed, with as many sense organs 


(tentacles, eyespots, statocysts) and nerve centres alternating 
with the lobes. There is no velum. The margin of the peri- 
stome is four cornered with the four corners prolonged into "oral 

lobes" or branching tentacular arms. 
There are four groups of gastric fila- 
ments projecting into the central gastric 
pouch and the radial canals branch 
many times. The gonads are four in 
number and are developed from the 
entoderm of the gastric cavity into 
which they project. The embryo is set 
free as a ciliated larva. It swims about 
for a time then becomes attached and 
develops into a hydra-like polyp. This 
polyp is called a scyphostoma and from 
it many medusae are formed by strobil- 
lation. The polyp becomes constricted 
just below the circle of tentacles and 
another circle of tentacles begins to 
form below this constriction. Then a 
third circle of tentacles and a third con- 
striction begin. This process continues 
downward while the polyp grows in 
length. The uppermost constriction 
grows continually deeper until a disc- 
shaped portion of the parent polyp 
with a margin of tentacles is completely 
cut off. There is thus formed a minute 
medusa which is called an ephyra. 

Later another disc is cut off and then another. By this method 
a number of ephyrae are produced and they develop directly 
into medusae. Sometimes the scyphostoma stage is omitted, 
the ciliated planula larva developing directly into the 

FIG. 134. Physalia, the 
Portuguese man-of-war. 
(From Galloway, after 



Order i. The Stauro medusae are a small group in which the sense 
organs are wanting. There is usually an aboral stalk for attachment. 

Order 2. The Lobomedusae are free swimming medusae with sense 
organs (tentaculocysts) in the notches between the lobes of the umbrella. 

FIG. 135. A small sea fan (Gorgonia) and a group of finger sponges. X 1/3. 

529. Class IV. Anthozoa. The Anthozoa comprise the sea 
anemones and corals. In this group the medusa stage is want- 
ing and the polyps are either solitary and comparatively large 
or smaller and colonial. They are especially abundant'in the 


warmer seas where they often occur in vast numbers. Some 
forms are widely distributed. To the Anthozoa, sponges and 
sea weeds, is chiefly due the brilliant coloration so often found 
in the sea bottom of tropical and sub-tropical seas. Most 
anemones are attached to some firm object such as rocks, sea 

FIG. 136. A whip coral (Gorgoniidse). X 1/2. 

weeds, shells or even the surface of other animals. This attach- 
ment is effected by a sucking action of the basal disc and permits 
the animal to move by a slow creeping motion. Some anemones 
lie patfly embedded in the sand into which they can completely 
withdraw by longitudinal contraction. Others form a leathery 



tube by a secretion of the column, still others secrete calcare- 
ous matter, especially from the surface of the basal disc. This 
is notably the case with the corals. Through this secretion 
the animal becomes immovably fixed. The basal disc of the 

FIG. 137. A branching madrepore coral, Astrangia. Slightly reduced. 

coral polyp secretes more rapidly along its edge so that a cup 
is formed into which the animal can more or less completely 
withdraw. Within this cup there are also vertical plates and 
pillars built up by the unequal secretion of the various parts 
of the base. To this is due the beautiful structure of many 



kinds of coral. Some corals secrete a horny skeleton and in 

some there is a mixture of the horny and calcareous substances. 

530. The larger anemones are considerably more complex 

FIG. 138. Coral colonies developing on a shell. Various steps in 
the process are shown. 

than the smaller corals but certain important characters are 
common to all and serve to distinguish this type of polyp from 
that of the Hydrozoa. The Anthozoan polyp is distinguished 
by the oesophagus and mesenteries. The mouth does not open 


directly into the gastro-vascular cavity as it does in the Hydro- 
zoan polyp. There is a long oesophagus which extends from 
the edge of the mouth to the centre of the gastric cavity. It 
is in reality a cylindrical continuation of the oral surface and is 
lined with ectoderm. The gastric cavity is incompletely 
divided into chambers by folds of the entoderm supported by 
layers of mesoglea. Some of these mesenterial folds extend 
from the wall of the column to the cesophagus. These are said 
to be complete. Others do not reach the cesophagus and are 
therefore known as incomplete mesenteries. On the free edge 
of some of the mesenteries there is a thick muscular cord, the 
mesenterial filament, which is richly supplied with gland and 
nettling cells. There are no special sense organs and nothing 
that can be called a central nervous system, though beneath the 
ectoderm of the oral disc the network of nerve fibres is better 
developed than elsewhere. A strong circular muscle is usually 
found just below the edge of the oral disc and in the mesen- 
teries there are strong longitudinal muscle bands. The gonads 
lie embedded in the mesoglea of the mesenteries along the free 
border. The sexes are usually distinct. The larva develops 
for a time within the body of the parent and escapes as a ciliated 
planula which becomes fixed and develops directly into the 

Order 2. The Octactiniaria are chiefly colonial. The polyp has eight 
pinnately branched tentacles and eight mesenteries. There is frequently 
a skeletal structure of horny or calcareous matter. The whip corals, sea 
fans, organ pipe corals, etc. 

Order 3. The Ceriantipatharia include the anemone Cerianthus and 
some colonial forms with a horny skeleton and polyps with six tentacles. 

Order 4. The Zoanthactiniaria comprise the sea anemones and the 
madrepore corals. The mesenteries are grouped in pairs. 

531. Class V. Ctenophora. The Ctenophora are another 
group of jelly fishes. They are more transparent and watery 
then the medusae and exceedingly fragile. A common type is 


the pear-shaped Pleurobrachia which is also comparable to a 
pear in size. The mouth is located at the small end and 
opposite it there lies a single statocyst. Extending about two- 
thirds of the distance from pole to pole and at about equal 
distances from the two poles are eight bands of vibratile plates 
which are regarded as rows of cilia fused together. These are 
the locomotor organs. In place of nettling cells the two 
long tentacles are covered with adhesive cells to which the 
prey adheres. 

532. The Ccelomata. In none of the Coelenterates are the fundamental 
animal characteristics strongly developed. Sense organs and the organs 
of locomotion are in no case highly developed and the symmetry of the 
body is always primarily radial though in some cases a tendency toward 
secondary bilateral symmetry may be observed. In the Ccelomata there 
is always a well-developed mesoderm. This makes possible a more highly 
developed muscular system and consequent greater locomotor activity. 
With this go also more highly differentiated sense organs and bilateral 
symmetry. The mesoderm is derived from the entoderm and encloses a 
paired series of cavities, the ccelomic, or body, cavity. 

533- PHYLUM III. Scolecida. Several classes of animals 
more or less resembling worms in the form of the body but with 
no evidence of metameric segmentation are often called the 
unsegmented worms. In this group the true body cavity is 
limited to small spaces connected with the excretory and re- 
productive organs. 

534. Class I. Platyhelminthes. The animals of this group 
have a flattened body. The digestive tract is sack-like, with- 
out vent, or wholly wanting. The space between the intestine 
and body-wall is filled with a parenchyma of contractile fibres. 
The nervous system consists of a paired supra-cesophageal 
ganglion and a pair of ventral longitudinal nerves. Two other 
pairs of longitudinal nerves are sometimes present. The 
excretory system consists of a branched system of protoneph- 
ridia, also called a water vascular system. The proximal 


end of the protonephridium is closed by a so-called flame cell. 
This is a large cell provided with long vibrating cilia which 
project into the proximal end of the canal. The flat worms are 
usually hermaphrodyte and the reproductive system is highly 

535. Order i. The Turbellaria, or gliding worms, are usually small, 
very much flattened, aquatic animals. The name refers to -the method 
of locomotion which is effected through the cilia by which the surface of 
the body is covered. The mouth is on the ventral side and is usually 
provided with an eversible proboscis. The digestive tract is a simple 
blind sack in the smallest microscopic forms, but in the larger ones it is 
divided into three main trunks which have numerous branches. This 
form of digestive tract is a substitute for a circulatory system. There are 
usually 2-many simple eyes at the anterior end and over the brain. 

536. Order 2. The Trematoda are parasites and consequently show 
more or less evidence of degeneration. In the form of the body they 
resemble the Turbellaria, but the surface of the body is destitute of cilia 
in the adult. The animal is provided with hold-fast organs in the form 
of hooks and suckers. Commonly there are two suckers, one at the 
anterior end enclosing the mouth and another further back on the ventral 
side. The digestive tract is usually two forked but may be more com- 
plexly branched. Eyes are only found in the ectoparasitic forms and 
in some free-living larval stages of endoparasites. The life history of a 
trematode is described in Part III. Page 366. 

537. Order 3. The Cestoda are all endoparasites and in the adult stage 
are found only in the digestive tract of higher animals. Special sense 
organs and digestive tract are both entirely wanting. The digested food 
of the host is absorbed through the surface of the body. In place of a 
head there is a hold-fast organ called a scolex which in the most common 
forms has a circle of four suckers and sometimes also a circlet of hooks. 
In a narrower region just below the scolex a process of strobilation takes 
place by which the body of the parasite is formed. This is usually com- 
posed of a long series of proglottides, the ones farthest from the scolex 
being the oldest. The last segments may be "ripe" while new ones are 
forming below the scolex. A pair of lateral nerves extend through the 
body from segment to segment. There is also a pair of longitudinal 
excretory tubes which are connected by transverse canals in each proglottis. 
Each proglottis contains also a complete set of reproductive organs highly 


developed, of both sexes. A "ripe" proglottis contains a large number of 
fertilized eggs. It is cut off from the main chain and passes from the host 
with the faeces. The embryonic stages develop in a second host as is 
described in the case of a typical example in Part III. Page 367. 

538. Class IT. Aschelminthes. The animals comprised in 
this class have usually a cylindrical body and a simple tubular 
digestive tract opening posteriorly by a vent. A false body 
cavity originating from the blastula cavity is often of consider- 
able size. The sexes are usually distinct. 

539. Order i. The Rotatoria are small, mostly microscopic, free-living 
animals. They are found chiefly in fresh waters. The anterior end of 
the body is provided with a contractile crown of cilia by which locomotion 
is effected. At the posterior end there is usually a stalk-like "foot" 
which is provided with adhesive glands. By means of this* foot the 
animal attaches itself temporarily. The name Rotifer has reference to 
the apparent revolution of the crown when the cilia are in motion. The 
currents produced by the cilia carry food particles to the mouth which 
lies in the centre of the crown. The oesophagus opens into a stomach 
which is provided with a set of cuticular teeth. A pair of excretory 
tubules opens into the posterior end of the intestine. 

540. Order 4. The Nematoda are in part free living, in part parasitic. 
They are often called thread worms or round worms. The mouth is at 
the anterior end. There is then a sucking oesophagus and a simple 
tubular intestine which opens on the ventral side near the posterior end. 
Special sense organs are practically wanting. There is a nerve ring around 
the mouth and from this a pair of nerves, one dorsal, one ventral, extend 
the length of the body. There is a pair of excretory tubules, one on either 
side, which extend from end to end of the body. The life histories of 
several parasitic forms are described in Part III. 

541. Class IV. Nemertini. This group is composed chiefly 
of free-living marine forms. The body is much elongated and 
muscular. The epidermis is ciliated. There is a long eversible 
proboscis and the intestine opens posteriorly by a vent. Eyes 
are often present and in large number and there is a pair of 
sensory grooves on the head. The nervous system consists of 
a supra-cesophageal ganglion and a sub-cesophageal ganglion. 


These are connected around the oesophagus and give off three 
longitudinal nerves, one dorsal and two ventral. There is a 
pair of branched protonephridia which open on the side of the 
body. The sexes are usually distinct. 

542. PHYLUM IV. Annelida. The true worms are free 
living, aquatic or, if terrestrial, at least confined to moist 
situations. The body is usually much elongated, bilaterally 
symmetrical and segmented, and the segments are similar 
(homonomous). The intestine is usually a straight tube with 
a vent at the posterior end of the body. There is a true body 
cavity completely lined with mesoderm. Eyes and other sense 
organs are often present. The nervous system consists of a 
supra-cesophageal ganglion, a pair of circum-cesophageal con- 
nectives and a ventral chain of ganglia arranged metamerically 
and connected by a pair of longitudinal nerves. In each seg- 
ment there is a pair of nephridia. 

543. Class n. Chsetopoda. The Chaetopoda include the 
typical worms, such as nereis and the earthworms. They are 
distinguished by the cuticular bristles or setae with which each 
segment of the body is armed. 

544. Order i. The Polychceta are marine annelids. They have two 
bundles of setae on each side of each segment. The setae are borne by 
short, unjointed appendages (parapodia) which are divided into two 
branches, each branch having a bundle of setae. They usually live on the 
sea bottom in burrows or tubes but some are pelagic. Many are active 
predatory animals and have well-developed sense organs. Others live in 
leathery or calcareous tubes formed by secretions of epidermal glands. 
These never leave the tubes voluntarily. Only the anterior end is in 
most cases protruded for the purposes of feeding and respiration. In 
many cases a circle of feather-like tentacles covered with cilia produce 
currents by which food is carried to the mouth. The sexes are usually 
distinct and there is a metamorphosis in development. 

545. Order 3. The Oligochata are the small fresh-water worms and the 
earthworms. They lack parapodia and the bristles are few in number, 
usually eight in each segment, two groups of two each on a side. They 
are hermaphrodytic and the development is direct. 


546. Class HI. Hirudinea. The leeches differ considerably 
from the Chaetopoda. The external segmentation does not 
correspond to the metamerism. There are usually three or 
five external rings to one segment. There are 34 metameres. 
The body cavity is almost obliterated by parenchyma tous tissue. 
They are all external parasites and are provided with suckers for 
holding to the host and for locomotion. There are two suckers, 
a small one at the anterior end enclosing the mouth and a large 
one on the ventral side at the posterior end. There are neither 
parapodia nor bristles. Series of lateral pouches render the 
digestive tract very capacious. The leeches are hermaphrody te. 
They are found only in the water, or in moist places. 

547. PHYLUN V. Molluscoidea. Under this head are 
grouped several classes which are sometimes placed under 
the heading worms. They are aquatic, usually fixed and 
provided with a cuticular covering in the form of a tube or a 
two-valved shell. They are unsegmented. The mouth is 
surrounded by a circle of tentacles covered with cilia. The 
intestine is usually U-shaped so that the vent lies near the 

548. Class n. Bryozoa. The Bryozoa, or Polyzoa, are all 
minute aquatic animals. Most are marine but there are also 
a few fresh- water forms. They are usually colonial and the 
colony may spread over considerable areas though the individ- 
uals are barely visible to the unaided eye. There is a gelatinous, 
horny or calcareous test into which the animal may completely 
withdraw. The tests of a colony often form an incrustation over 
the surface of other objects and again they form branching 
plant-like structures comparable to moss plants in size and 
general appearance, hence the name. The test is secreted by 
the epidermis and forms part of the animal. A pair of strong 
retractor muscles cause the rapid withdrawal of the body 
completely within the test on the slightest irritation. The 
mouth is surrounded by a crown of ciliated tentacles, sometimes 



FIG. 139. Amphitrite ornata, a marine Polychaete. (From Galloway, 

after Verrill.) 

FIG. 140. Cirratulus grandis, a marine Polychaete. (From Galloway 
after Verrill.) 


borne on a horseshoe-shaped disc, the lophophore. The vent 
lies just outside the tentacles. There are no special sense organs. 
The nervous system consists of a single ganglion and a few 
nerves leading from it. The nephridia are very rudimentary or 
entirely wanting. The colonies ate often polymorphic; certain 
individuals being specialized receptacles for eggs while others 
form slender, jointed, vibrating whips or a pair of jaws which 
open and close like the beak of a bird. 

549. Class HI. Brachiopoda. The Brachiopods are not 
generally well known, although they are not at all rare and are 
widely distributed. They live only in salt water and may be 
found not far from shore. They have a shell somewhat like 
that of a clam but the plane of symmetry cuts the hinge at 
right angles and one of the valves is dorsal and the other ventral. 
They are usually unlike. The shell is composed either of chitin 
or calcareous matter. The animal is attached by a stalk-like 
extension of the posterior end of the body or by the cementing 
of the ventral valve to the substratum. On either side of the 
mouth the body is prolonged into a pair of coiled arms which 
bear numerous ciliated tentacles. There is a true body cavity 
in which lie the simple U-shaped digestive tract with a digestive 
gland, the heart, the gonads, one or two pairs of nephridia and 
a dorsal and a ventral ganglion. 

550. PHYLUM VI. Echinodermata. This is a well-defined 
group of animals. They are all marine, without exception. 
The larvae are bilateral but by a metamorphosis the adult 
becomes radially symmetrical and the number of rays is 
usually five. The name, Echinoderm, means spiny skin and 
refers to the calcareous bars and plates embedded in the integu- 
ment. These are not equally well developed in the various 
classes but they serve in varying degree as an exoskeleton and 
in most forms some pieces project beyond the general surface 
in" the form of spines. The ambulacral system is the most 
distinctive anatomical character of the group. It consists of a 



system of tubes filled with water which enters through the 
sieve-like madreporic plate near the aboral pole of the animal. 
From here a " stone canal" leads to a ring canal which circles 
the mouth and gives off five radial canals to the five rays of the 
body. Each radial canal has numerous lateral branches which 
end in tubular processes of the integument, called tube feet. 

FIG. 141. Starfish, oral view showing tube feet. (From Galloway, 
after Leuckart and Nitsche.) 

At the base of each foot there is a bladder-like lateral enlarge- 
ment of the foot canal, the ampulla. By its contraction the 
ampulla forces the water into the foot causing it to elongate. 
When the foot contracts the water is forced back into the 
ampulla. The end of the foot is provided with a sucking disc 
by which it may be attached to an object. By this mechanism, 



FIG. 142. Section through the arm and disc of a starfish. A, Anus; amp, 
ampulla; cb, circular blood-vessel; civ, circular water canal; co, ccelom; co.e., 
ccelomic epithelium; d.b., dermal branchiae; e, eye-spot; ect, ectoderm; ent, ento- 
derm; /, ambulacral foot; g, ambulacral groove; h, hepatic caeca; i, intestine; 
i.e., intestinal caeca; mes, mesoderm; mo, mouth; mp, madreporic body; nr, 
nerve ring; os, ossicles; rn, radial nerve; rb, radial blood-vessel; rp, genital pore; 
rw, radial water canal; sc, stone canal; sp, spine. (From Galloway.) 

d. b. 



FIG. 143. Cross section of the arm of a starfish, ar, Ambulacral rafter; ov, 
ovary containing ova. Other lettering as in preceding figure. (From 
Galloway. ) 



the numerous feet acting in unison may slowly pull the animal 
along. The spines frequently assist in locomotion, in which 
case they are long, jointed at the base and operated by special 
muscles. In some cases the tube feet lack the sucking disc and 
serve either as sense organs or 
organs of respiration. 

There is a b'ody cavity in 
which the digestive and repro- 
ductive organs are freely sus- 
pended. The circulatory system 
is not well developed. Special 
sense organs of a simple type 
sometimes occur. The nervous 
system is also poorly developed, 
consisting chiefly of a ring of 
nerve fibres encircling the mouth 
and sending a radial nerve into 
each ray of the body. 

551. Class I. Pelmatozoa. 
The crinoids or "sea lilies" are 
the only living representatives 
of this group. The body is at- 
tached by a stalk-like develop- 
ment of the aboral pole. In a 
few forms this is true only in 
the young stages but for most 
families the stalked condition is 
permanent. The skeletal ele- 
ments are highly developed. The stalk, body and arms are 
largely composed of calcareous joints and plates. The five rays 
are usually repeatedly branched. The ambulacral system 
consists of ciliated tentacles which serve for respiration and to 
maintain the currents by which food is carried to the mouth. 
In connection with the latter function, ciliated grooves on the 

FIG. 144. Antedon, a Crinoid. 
X 1/2. 


oral surface of the arms are also important. The digestive 
tract opens by a vent on the oral surface in an eccentric 

552. Class II. Asteroidea. The starfishes lie on the sea bot- 
tom with the mouth or oral side down. The ambulacral feet 

FIG. 145. A starfish, Astropecten. X 3/4. 

serve for locomotion. The oral-aboral axis is shorter than a 
radius and the five radii are longer than the inter-radii so that 
the body assumes the form of a five-rayed star. The skeleton 


consists of small rod-shaped pieces attached to each other by 
muscles in such a way as to form a network. This skeleton is 
very flexible and by means of the muscles the arms may be 
slowly bent through an arc of 180 or more. Short spines, 
more or less movable, project beyond the general surface. 
Each radial canal ends at the tips of the arm in a tentacle 
and at its base there is an eyespot. 

553. The starfish is carnivorous, living largely on shell fish. 
The mouth is located at the centre of the disc and is capable of 
opening to an enormous size so that large objects can be taken 
into the stomach. After digestion the insoluble part is ejected 
at the mouth. The animal also attacks larger prey than it 
can swallow. By attaching some of the tube feet to the solid 
substratum and others to the two valves of a shell fish the 
shell may be pulled open. Then another remarkable feat is 
performed: The stomach is everted though the mouth and 
its rather voluminous folds are thrown around the soft parts 
of the prey. Digestion then takes place outside the body of 
the animal. Five pairs of long retractor muscles connect the 
wall of the stomach with the interior of the arms; their function 
is to draw the stomach back into the body. The stomach is 
connected with a vent at the aboral pole by a small intestine. 
But intestine and vent are largely functionless and are often 
rudimentary. Connected with the stomach are five long 
tubes which project into the cavities of the arms. The walls 
of these tubes are glandular and secrete a digestive fluid. 
These glands are called hepatic caeca. The sexes are distinct 
and the five pairs of gonads lie in the body cavity, a pair in each 

554. Class HI. Ophiuroidea. The serpent stars, or brittle 
stars, have the general form of the starfish but are more dis- 
tinctly divided into disc and arms. The arms are propor- 
tionately longer and more slender and also more actively 
movable and it is entirely by the sweeping movement of the 


arms that locomotion is effected. The ambulacral "feet" are 
without sucking discs and ampullae and do not assist in loco- 
motion. The arms are almost solid, that is, the digestive and 
reproductive organs do not extend into them. The skeletal 
parts are better developed than in the starfish. In the "basket 
fish" the arms are branched. 

FIG. 146. A brittle star (Ophiuroidea). X 3/4. 

555. Class IV. Echinoidea. In the sea urchins the radii 
and inter-radii are almost or quite equal and hence there are no 
"arms." The skeletal parts are broad plates which fit together 
so as to form a single piece, the shell or test. The oral surface 



is down and the oral aboral axis is often nearly equal to a 
horizontal diameter so that the body is nearly spherical. The 
spines are often long and are movable and are important in 
locomotion. The tube feet are much as in the starfish. The 
true sea urchins are vegetable feeders and the mouth is pro- 
vided with a set of five jaws which form a complicated ap- 
paratus known as Aristotle's lantern. The intestine is long 

FIG. 147. The basket fish, Astrophyton. X 1/2. 

and coiled and opens by a vent on the aboral surface. In 
some groups of Echinoidea a secondary bilateral symmetry 
appears and in these the vent is at the posterior margin of 
the oral surface. 

556. Class V. Holothuroidea. The sea cucumbers have 
the principal axis horizontal so the oral-aboral axis is at right 



angles to that of the other Echinoderms. The principal axis 
is also much longer than the radii. The skeletal parts are re- 
duced to 'minute hooks and plates, but the integument is very 
thick and leathery. The animals feed on organic detritus 
which they collect by means of a circle of branching tentacles 

FIG. 148. A sea-urchin (Clypeaster). The spines have been removed. The 
five ambulacral areas are clearly shown. The test shows marked bilateral 

surrounding the mouth. The intestine is a coiled tube and 
ends at the aboral pole of the animal in a large cloacal cavity. 
Lying in the body cavity and connected with the cloaca is a 
very peculiar organ called the respiratory tree. It is a tubular 


structure and is many times branched. The main stem opens 
into the cloaca. The respiratory tree is filled with water which 
is regularly renewed from outside through the cloacal vent. 
This is brought about in the following way. Numerous small 
muscles connect the outside of the cloacal chamber with the 
body wall. When these contract the cloaca expands and fills 
with water. The cloacal aperture then closes, the walls of the 
cloaca contract and the contents are forced into the respiratory 
tree. The tubes of the respiratory tree also have contractile 
walls and these by contraction again force the water out. In 
some of the small Holothuria, which do not possess such an 
impervious integument, the respiratory tree is absent. The 
ambulacral system is variously developed within the group. 
In several families the tube feet are entirely wanting. Loco- 
motion is chiefly effected by worm-like movements of the body. 

557. PHYLUM VII. Arthropoda. The Arthropoda are bi- 
laterally symmetrical, segmented animals. They are distin- 
guished from the Annelida by their jointed appendages. The 
number of segments is usually not more than twenty, and they 
are not alike (heteronomous). The body is always covered 
with a cuticula of chitin, secreted by the epidermis. One or 
more pairs of appendages are modified to serve as mouth parts 
for the ingestion of food. The blood vessels open into the body 
cavity which is also connected with the cavities derived from 
the primitive blastula cavity. The body is typically divided 
into three regions, head, thorax and abdomen. The nervous 
system consists of a brain and ventral nerve cord as in the Anne- 
lids. In point of numbers the phylum includes two-thirds of 
the animal kingdom. 

558. Class I. Branchiata. As the name implies the 
Branchiata are the Arthropods which are provided with gills. 
But this is true only of the larger forms and even in some of 
these the gills have been lost. With a very few exceptions, 
however, the Branchiata are aquatic. The term Crust'aceae 


is also frequently applied to the group because the chitinous 
cuticula is impregnated with salts of lime which render it very 
hard. The appendages are forked (biramous) and at least 
three pairs are modified as mouth parts and two pairs, the 
antennae, have a sensory function. 

559. The orders Phyllopoda, Ostracoda, Branchiura and Copepoda com- 
prise only small forms, seldom more than an inch in length and usually 
minute. The Phyllopoda are characterized by their broad, leaf-like swim- 
ming appendages. The Ostracoda have a carapace in the form of two 
valves, like the shell of a clam, which can be opened and closed. The 
Copepoda are cigar shaped and have a single median eye. The Ostracoda 
and Copepoda are very common in our fresh-water ponds. The Bran- 
chiura are parasitic in the gill chambers of other Crustacea and on fishes. 
There are also many parasites among the Copepoda. 

FIG. 149. A shrimp, Palaemonetes vulgaris. (From Galloway, after Verrill.) 

560. Order 5. The Cirripedia are the barnacles. They are all marine. 
The young are free swimming but they come to rest and attach them- 
selves to some object. A series of calcareous plates are formed by folds 
of the skin. Some of these are hinged and can be moved by muscles. 
The animal may be entirely enclosed by the shell. The appendages are 
long and slender and are fringed with hairs. These organs are thrust out 
into the water and by a sweeping motion currents carrying food particles 
are directed toward the mouth. The eyes are degenerate in the adult 
but the reproductive organs are highly developed. The barnacles are 
usually hermaphrodytic. Several families belonging to this group are 
parasitic. The sacculina described in Part III is a notable example. 


561. Order 6. The Malacostraca are a large group, comprising all the 
larger Crustacea. The head and thorax together are composed of thirteen 
segments and the abdomen of six. There are always two pairs of antennae, 
a pair of mandibles and two pairs of maxillae. In most cases several more 
pairs of appendages function as mouth parts. This order is very large 
and includes shrimps, prawns, crayfish, lobsters, crabs, sand fleas and 
"wood-lice." Of the five great divisions of the group the following are 
the most important. Legion 2. Thoracostraca. The compound eyes are 
on movable stalks and the head and most or all of the segments of the 
thorax are covered by a single cuticular shield. The gills are usually 
attached to the basal joints of the thoracic appendages or to the adjacent 
parts of the body-wall and are covered by the cephalo-thoracic shield. 

FIG. 150. Caprella geometrica, an Amphipod. (From Galloway, after Verrill.) 

The sub-order Decapoda comprises the most important families. In this 
group the five pairs of appendages from the gth to i3th segments inclusive 
are ambulatory appendages. Those of the 3rd to 8th are mouth parts. 
The following analysis of the classes of the malacostraca will show the 
relation of the groups mentioned. 
Order, Malacostraca. 

Legion i. Leptostraca. 
Legion 2. Thoracostraca. 

Sub-order i. Schizopoda. 
Sub-order 2. Decapoda. 

Section i. Macrura natantia, shrimps, pawns. 
Section 2. Macrura reptantia, crayfish, lobster. 
Section 3. Anomura, hermit crabs. 
Section 4. Brachyura, crabs. 
Sub-order 3. Cumacea. 
Legion 3. Stomatopoda. 
Legion 4. Anomostraca. 
Legion 5. Arthrostraca. 

Sub-order i. Anisopoda. 

Sub-order 2. Isopoda, "pill bug" = " wood-louse." 

Sub-order 3. Amphipoda, sand fleas. 


562. The Macrura natantia (swimming large tails) are generally com- 
pressed laterally, the ambulatory appendages are slender and the animal 
depends chiefly on the backward stroke of the strong abdomen for loco- 
motion. The Macrura reptantia (crawling large tails) are not compressed 
and the thoracic appendages are strong as is also the abdomen. The 
section Anomura includes the hermit crabs which live habitually with 
the abdomen thrust into an empty snail shell. For this reason the 
abdomen is twisted, the posterior thoracic appendages reduced and the 
caudal fin transformed into an unsymmetrical hold-fast organ. The 
Brachyura are the true crabs in which the abdomen is much reduced and 
turned forward under the cephalo-thorax. 

563. The Arthrostraca have sessile eyes and the thoracic shield is almost 
or wholly wanting. In the Isopoda the body is flattened dorso-ventrally 
and there are no gills on the thoracic appendages. The Amphipoda are 
compressed laterally and gills are present on the thoracic appendages. 
The body is curved with the abdomen turned down and forward. The last 
appendages of the abdomen are turned backward and are used in connec- 
tion with the backward stroke of the abdomen to spring the body forward 
analogous to the movement of a flea, hence the name. Most of the 
Malacostraca are marine. The crayfishes are fresh-water representatives 
of the order. A number of crabs live on land though usually in the 
vicinity of water. The wood-louse is also a familiar terrestrial form, 
though it is also at home only in damp places. The terrestrial crabs have 
rudimentary gills and the gill chamber takes on the function of a lung. 

564. Class n. Palaeostraca. These arthropods have only 
one pair of appendages anterior to the mouth and five pairs 
surrounding the mouth. Those around the mouth serve both 
for the ingestion of food and for locomotion. 

565. Order 2. The Xiphosura have a broad horseshoe-shaped cephalo- 
thorax, an unjointed abdomen and a long spine-like terminal appendage, 
to which the name "sword tail" refers. There are a pair of ocelli and a 
pair of compound eyes on the dorsal surface of the cephalothorax. The 
first pair of appendages are chelate but are not used in locomotion. The 
following five pairs are locomotor but their basal joints are spiny and 
serve for the trituration of the food. The abdominal appendages are 
broad leaf -like structures which protect the "book gills" which are 
attached to them. There is only one genus of Xyphosura, the horseshoe 
crab, Limulus. 




FIG. 151. Limulus, the horse-shoe crab. Dorsal view. (From Patten.) 



566. Class HI. Arachnoidea. The Arachnoidea are air- 
breathing Arthropods, with a cephalothorax, two pairs of 
mouth appendages and four pairs of locomotor appendages. 
The abdomen has no appendages. There are from 2 to 1 2 eyes. 

567. Order i. The Scorpionidea are large Arachnoidea with a long 
segmented abdomen consisting of a preabdomen of seven segments and 

FIG. 152. A Scorpion, Buthus afer. X 1/2. 

a postabdomen of six segments and ending in a spine and poison gland. 
The first and second pairs of appendages, the chelicerae and maxillary 
palps are chelate. The respiratory organs consist of four pairs of book 
lungs which open on the ventral side of the preabdomen. 

568. Order 2. The Pedipalpi have clawed chelicerae, clawed or chelate 


maxillary palps; and the third pair of appendages are whip like. The 
abdomen is 11-12 jointed. There are two pairs of book lungs. 

569. Order 3. The Araneida, or true spiders, have the abdomen con- 
nected with the cephalothorax by a narrow waist. The abdomen is 
usually unsegmented. The chelicerae are chelate and the maxillary palps 
are similar to the ambulatory appendages. There are four book lungs 
or, two book lungs and two tracheae or, four tracheae. On the ventral 
surface of the abdomen is a group of four or six glands from which the 

FIG. 153. The great bird-killing spider, Mygale, of South America. X 5/8. 

web is spun. The web is a fluid secretion which sets on exposure to the 
air. It is used to build and line the nest, for building traps by which 
the prey is caught, for winding about the egg masses, for locomotion and 
a variety of other purposes. The chelicerae are provided with a poison 

570. Order 6. The Opilionidea are the "harvestmen" or " daddy-long- 
legs." The abdomen is broadly connected with the cephalo-thorax so 
that the whole forms apparently one continuous body. The chelicerae 
are chelate and the maxillary palpi are like the legs. The legs are often 
very long. The respiratory organs are tracheae. 



FIG. 1 54;! . Home of the trap-door spider. Door closed. 

FIG. I54-B. Home of the trap-door spider. The door propped open 
with a straw. X i. 



571. Order 8. The Acarina are the mites and ticks. Many of the 
members of this group are parasites. The body cannot be divided into 
cephalo-thorax and abdomen as all evidence of segmentation is lacking. 
The mouth parts are often adapted for piercing and sucking. The legs 

FIG. 155. The home of the trap-door spider laid open. The door is held open 
with a pin. X 3/4- 

are often merely hold-fast organs. The parasitic forms are often much 

572. Order 9. The Linguatulida are parasitic forms especially notable 



for the degenerated condition of many organs. The body is worm-like 
and the appendages are reduced to two pairs of hooks. 

573. Class 6. Protracheata. This group is of interest as 
forming a connecting link between the annelids and insects. 

FIG. 156. The trap-door spider. X i. 

There are only a few species, which are not common but are 
found in widely separated parts of the globe. The animals are 
small, worm-like, and are found in damp places under stones, 
decaying wood or other similar situations. The body is worm- 

FIG. 157, Peripatus capensis, an example of the class Protracheata. 
(From Galloway, after Moseley.) 

like and provided with short feet somewhat like the false feet 
of a caterpillar but provided with claws. There is one pair of 
tentacles and a pair of eyes. There are two pairs of appendages 
in the region of the mouth. The respiratory organs are tracheae 
with stigmata scattered irregularly over the surface of the body. 



574. Class 7. Myriapoda. The "thousand legs" have a 
worm-like body divided into similar segments, a distinct head 
with one pair of antennae, usually one pair of maxillae and with 
one or two pairs of appendages on each body segment . Respira- 
tion is by tracheae and the stigmata are arranged segmentally. 

575. Order 3. The Diplopoda are the 
common "thousand legs." The body is 
cylindrical or half-cylindrical and is 
covered with a chitinous cuticula hardened 
by deposits of carbonate of lime. All the 
segments except a few of the most anterior 
and the last one bear two pairs of ap- 
pendages. The animals are vegetable 
feeders and harmless. 

576. Order 4. The Chilopoda bear some 
resemblance to the preceding group but 

FIG. 158. Spirobolus, a Diplopod. 
(From Folsom.) 

FIG. 159. Campodea, an example 
of the class Apterygogenea. (From 

the body is usually flattened and no segment bears more than one pair of 
appendages. There is a pair of mandibles, two pairs of maxillae and one 
pair maxillipeds. The maxillipeds belong to the first body segment. They 
are stout claws and contain a poison gland. The centipede of the south 
is an example of this order but a more familiar one is the long-legged 
Cermatia often seen in our dwellings where it preys upon other insects. 



577. Class 9. Apterygogenea. This class is often grouped 
with the insects as the lowest order. They have many char- 
acters in common with insects and are doubtless closely related. 

FIG. 1 60. Mouth parts of a cockroach, Ischnoptera pennsylvanica. A, 
labrum; B, mandible; C, hypopharynx; D, maxilla; E, labium; c, cardo; g (of 
maxilla) galea; g (of labium) glossa; /, lacinia; Ip. labial palpus; m, mentum; 
mp, maxillary palpus; p, paraglossa; pf, palpifer; pg, palpiger; s, stipes; sm, 
submentum. B, D and E are in ventral aspect. (From Folsom.) 

FIG. 161. Alimentary tract of a grasshopper, Melanoplus. c, Colon; cr, crop; 
gc, gastric caeca; i, ileum; m, midintestine, or stomach; ml, malpighian tubules; 
o, oesophagus; />, pharynx; r, rectum; s, salivary gland. (From Folsom.) 

There is a distinct head, a thorax of three segments and ai 
abdomen of 6-1 1 segments. The head bears a pair of antennae 



and sometimes compound eyes. An ocellus may also be pres- 
ent. There is a mandible and two pairs of maxillae. Each 
segment of the thorax bears a pair of appendages. The only 
evidence of abdominal appen- 
dages to be found are a pair of 
stiff bristles attached to the 
ventral surface of the fifth 
segment and projecting forward 
beneath the body, and a pair of 
hooks beneath the third seg- 
ment. By means of this ap- 
paratus the animal makes 
springing movements. Hence 
they are called spring tails. The 
respiratory organs are tracheae 
except in some groups where 
respiratory organs are wanting. 

578. Class 10. Insecta. In 
the Insects the body is divided 
into head, thorax of three seg- 
ments (prothorax, mesothorax 
and metathorax) and an ab- 
domen of 9-10 segments. The 
appendages of the head are a 
pair of antennae, a pair of man- 
dibles, a pair of maxillae and 
a labium which represents a 
second pair of maxillae. Each 
segment of the thorax bears a 
pair of legs and in addition 
the meso- and metathorax also 

each have a pair of wings. The abdominal segments bear no 

579. The mouth opens into a narrow oesophagus with which 

FBG. 162 Tracheal system of an 
insect, a, Antenna; b, brain; I, leg; 
n, nerve-cord; p, palpus; s, spiracle; 
st, spiracular, or stigmatal branch; 
/, main tracheal trunk; v, ventral 
branch; vs, visceral branch. (From 
Folsom, after Kolbe.) 


several salivary glands are connected. The oesophagus is often 
enlarged to form a crop. In several orders there is also a 
gizzard between the crop and the stomach. The stomach is 
the true digestive portion of the alimentary canal. It is larger 
in diameter than oesophagus or intestine and usually has sack- 
like or tubular glands opening into its anterior end. Following 
the stomach is first a narrow small intestine and then a wider 
large intestine. At the junction of stomach and small intestine 
there are a number of long and very slender tubes, known as 
Malpighian tubules. These are the excretory organs of the 
insect. The respiratory system consists of a greatly branched 
system of trachea. These open on the surface at the side of the 
abdomen and thorax. The tracheal capillaries extend to all 
parts of the body. The heart is a long contractile vessel lying 
on the dorsal side of the abdomen. The blood enters it through 
eight pairs of ostia segmentally arranged. A vessel leads 
forward from the heart into the head. The blood circulates 
through the body cavity. 

580. There are usually a pair of highly developed compound 
eyes and sometimes two or three ocelli. Other special sense 
organs are the tactile hairs on the antennae, the olfactory cones 
and pits of the palpi and taste cells of the mouth cavity. The 
"brain" is large and complex in structure. The ventral gang- 
lionic chain may in reality be a chain of as many as twelve 
ganglia, but various stages of concentration occur even to the 
fusion of all into one mass. 

581. The gonads open by a pair of ducts at the posterior 
end of the abdomen. The sexes are separate and usually di- 
morphic. In some orders polymorphism is not uncommon. 
The eggs, in many cases, develop without fertilization (par- 
thenogenesis) . In most orders there is a marked metamorphism. 

582. Order i. The Orthoptera are insects with biting mouth parts, two 
pairs of wings which are unlike, and development by an incomplete meta- 
morphosis. The order includes, earwigs, cockroaches, the praying- 


mantis, "devil's horse" or "darning needle," grasshoppers, katydids and 

583. Order 3. The Corrodentia have biting or rudimentary mouth parts, 
wings alike, and development without or with little metamorphosis. The 
group includes the highly interesting "white ants" or termites and the 
less interesting body lice, ectoparasitic on mammals. The latter are 
degenerate, lacking wings and having rudimentary mouth parts and eyes 
greatly reduced. The termites are colonial and polymorphic. The sexu- 
ally perfect males and females are winged but the wings are later cast off. 
A third form called a worker and sometimes another form called a soldier 
may also be found. These are individuals in which the reproductive 
system remains undeveloped. 

584. Orders 6 and 7. The Odonata, "damsel flies," dragon flies, and 
Ephemeroidea, "day flies," are found only in the vicinity of fresh-water 
ponds or streams in which the larval development takes place. 

585. Order 8. Some of the Neuroptera, "lace wings," also develop in 
the water as for example Corydalis whose larva is the "hellgrammite." 
The ant lion ("doodle bug") is the larva of another "lace wing." The 
Odonata have an incomplete, the Ephemeroidea and Neuroptera a complete 
metamorphosis. All the remaining Orders except the last, Rhynchota, 
also undergo complete metamorphosis. 

586. Order n. The large order Lepidoptera, butterflies and moths, is 
perhaps the most readily distinguished of all. Here the maxillae are 
modified for sucking and form a proboscis. The two pairs of wings are 
similar and covered with scales. The prothorax is united firmly with the 
mesothorax. The larva is a caterpillar with distinct head and jaws devel- 
oped for biting. The head also bears two antennae and two or three pairs 
of simple eyes. The first three segments behind the head have jointed 
appendages and there are besides on the segments of the abdomen from 
two to five pairs of false feet. After a time of voracious feeding and rapid 
growth the larva attaches itself in some sheltered place or spins a cocoon 
of silk fibres. It then undergoes a complete change of form, meta- 
morphosis, becoming a quiescent pupa in which condition it continues 
for a short time if it is in the summer or through the winter if pupation 
takes place in the fall. Finally another transformation takes place, the 
integument of the pupa bursts and the imago emerges in all respects a 
mature insect. The life period of the imago is usually brief; the female 
is fertilized, deposits a single brood of eggs and dies. 

587. Order 12, The Diptera or two wings include the flies, gnats, and 
mosquitos. They have mouth parts developed for sucking or piercing. 




The anterior wings are membraneous, the posterior pair reduced to "bal- 
ancers" or halteres. The body is usually compact with the ventral chain 
of ganglia united into a single mass. The 
abdomen consists of 5-9 segments. The larva 
is a footless and often headless grub (maggot) 
and in the process of metamorphosis is trans- 
formed into a pupa and finally the imago. 
The larvae of the mosquitos are aquatic, those 
of most of the true flies live in decaying organic 
matter but many are parasitic. In a number 
of cases the adult is also parasitic. 

588. Order 13. The Siphonaptera or fleas. 
In this group the wings are wanting through 
degeneration. Compound eyes are also lack- 
ing. The body is laterally compressed. The 
mouth parts are for piercing and sucking. 
The third pair of legs are used for springing. 
The larvae are usually free living, the adult an 
external parasite. 

589. Order 14. The Coleoptera, or beetles, 
are a very large order. The mouth parts are 
constructed for biting. The anterior pair of 
wings are horny, the second pair membraneous. 
The first pair are called elytra. They fit 
together to form* a shield over the abdomen 
and at rest the second pair of wings are folded 
under them. The larvae have a distinct head 
with simple eyes and a soft body ("grub 
worms"). The feet may be wanting. The 
grub lives in protected situations, underground, 
under the bark of trees or boring into wood or 
in other similar places. Metamorphosis in- 
cludes a pupa stage. 

590. Order 16. The Hymenoptera include 
the ants, bees and wasps. The mouth parts 
are adapted for biting. The wings are two 

There are 

FIG. 163. The Lantern- 
fly of Brazil. Fulgora 
lanternaria. This odd ex- 
ample of the Rhynchota 
is said by the natives to 
carry a light in the pecu- 
liar appendage borne on 
the head. This statement pairs, of a membraneous texture. 

is seriously questioned, two compound eyes and three ocelli. The 
ofCt h n e Tr females are provided with a sting of a complex 
known. X 5/4. structure and located at the posterior end of 


the abdomen. This may be used for depositing eggs or for defense. 
The brain is highly developed. Some larvae feed on leaves, others are 
parasitic in the tissues of other insects or of plants while others are 
fed by the adults with either animal or vegetable food. The larvae 
usually spin a cocoon in which the pupa stage is passed. Some of the 
most important forms are: The gall wasps which deposit the eggs in 
the tissues of plants whereby a gall develops and forms a shelter and 
source of food for the larvae; the ichneumon flies which sting the 
larvae of other insects and deposit their eggs there, the larvae then 
developing as internal parasites; the ants with their complex social organi-r 
zation, polymorphism, consisting of three or four types of individuals, 
and division of labor, keeping of slaves, cultivation of plants and fostering 
of aphids for economic purposes; the common solitary wasps and social 
wasps with the more or less artfully constructed nests of mud or " paper"; 
the social and polymorphic honey bee and the bumble bees with their 
combs and honey. This interesting order merits a special treatise. 

591. Order 17. The Rhynchota or bugs. These insects are provided 
with a protruding snout and piercing mouth parts. Metamorphosis 
occurs in some cases, in variable degree. The wings are sometimes 
wanting but there are usually two pairs. The anterior pair may be 
partly horny. They are all ectoparasitic on other animals or plants. 
Included in this group are the bed bugs, the plant bugs, such as the 
squash bug, chinch bug and cicada, the water bugs, water-boatmen, 
water-striders and electric-light bug, and the "plant lice" and scale 

592. PHYLUM VIII. Mollusca. The Molluscs are the highest 
group of unsegmented animals. The group is pretty well 
denned but there is a great difference in the scale of organization 
between the lowest and highest orders. The most marked 
anatomical character of the phylum is the mantle, which is a 
single or paired fold of the integument of the dorsal side of the 
body. This mantle is usually of sufficient extent to entirely 
enclose the body of the animal, and on its external surface it 
secretes a hard shell composed of horny and calcareous matter 
deposited in layers. There are no paired appendages. A 
ventral muscular portion of the body is called the foot and 
sometimes serves for locomotion. The nervous system con- 


sists of three ganglia called the cerebral, pedal and visceral 
ganglia. .These are connected by paired nerves. The ccelomic 
cavity is almost obliterated by a mesenchymatous parenchyma. 
A pair of nephridia connect the remnant of the body-cavity 
with the exterior. There is also a pair of gonads. The body 
is fundamentally bilaterally symmetrical but in one large group 
a twisting of the visceral mass results in more or less asymmetry. 
The mollusca are primarily aquatic animals but a large number 
have become adapted to a terrestrial life. 

593. Class I. Amphineura. This is a small class and only 
one example need be described. Chiton is bilaterally sym- 
metrical and flattened dorso-ventrally. There is a partially 
differentiated head but the mantle fold includes the head 
as well as the body. On its dorsal surface the mantle forms a 
single series of eight plates. The foot is very broad and mus- 
cular and is used for locomotion and as a sucking disc for a 
hold-fast. In the groove between the mantle and the side of 
the body is a series of ctenidia, or comb-like gills. The digestive 
tract consists of a mouth cavity with radula and a pair of 
" salivary" glands, an oesophagus, stomach with a pair of 
digestive glands and a coiled intestine. The vent is at the 
posterior end, opening into the mantle cavity. The nervous 
system consists of an cesophageal ring or nerve collar and two 
pairs of longitudinal nerves, one ventral and one lateral. Be- 
sides, there are several smaller ganglia and numerous connectives 
between the longitudinal nerves. There is a pair of nephridia 
and a double gonad with paired ducts. All the Amphineura 
are marine. 

594. Class IT. Conchifera. This group is distinguished 
from the preceding by the fact that the mantle fold does not 
include the head and by the way in which the shell is formed. 
The latter consists of numerous spine-like pieces in the Amphi- 
neura while in the Conchifera it is formed in layers as a single 


595. Order i. The Gastropoda have a distinct head, a twisted visceral 
sack, a single shell and a creeping foot with, sometimes, lateral swimming 
lobes. The head usually bears two or four tentacles and a pair of eyes. 
The visceral organs are chiefly contained in a dorsal conical sack-like 
development of the body-wall which is more or less coiled in form of a 
spiral. The lower edge of the mantle forms a collar-like continuation of 
the ventral edge of the visceral sack. The entire surface of the visceral 
sack down to the edge of the collar is covered with a calcareous shell into 
which the head and foot may also be withdrawn. Between the surface 
of the mantle and the body there is a space of considerable size. This is 
called the mantle cavity. In it lie two feather-like gills, ctenidia, which 
are developed from the^body-wall. Because of the twisting of the vis- 
ceral mass the gills become shifted in position and one is frequently 

FIG. 164. The garden snail, Helix. A, The shell in section, a, Apex; an, 
anus; ap. aperture; c, columella; e, eyestalk; /, foot; /, lip; m, edge of mantle 
(collar); ra, respiratory aperture; s, suture; /, tentacles. (From Galloway.) 

596. In the mouth cavity there is a horny jaw and a tongue covered 
with a rough, file-like cuticular ribbon. This as called a radula. The 
digestive tract is rather long and coiled. There are a pair of "salivary" 
glands with ducts opening into the buccal cavity. The oesophagus is 
enlarged into a crop. Another enlargement of the canal forms a stomach 
into which the ducts of a large digestive gland ("liver") open. The 
coiled small intestine opens into a shorter and wider large intestine. 


The vent is usually on the right side anterior to the visceral mass. The 
heart consists of a ventricle and one or two auricles. It lies in a small 
body cavity called a pericardial chamber. The kidney communicates 
with the pericardial chamber by a nephridial funnel and opens into the 
mantle chamber through a duct the ureter. 

597. The nervous system consists of a pair of cerebral ganglia, pleural 
ganglia and pedal ganglia which are all closely connected into a nerve 
collar. There are also parietal and buccal ganglia. From these ganglia 
nerves are supplied to the sense organs and muscles of the head, to the 
mouth, to the foot, to the gills, olfactory organs (osphradia) and a part 
of the mouth, and to the buccal mass and intestine respectively. The 
sense organs usually present are tentacles, eyes, a statocyst which is 
usually close by the pedal ganglion though it is innervated from the brain, 
and chemical sense organs, called osphradia, located on or near the gills. 

598. Some of the Gastropods are hermaphroditic, in others, the sexes 
are separate. The reproductive system is frequently very complicated 
for besides the gonads and their ducts which may be variously modified, 
there may be two, three or more kinds of glands and other accessory 
reproductive organs. Development is either direct or by metamorphosis. 
The embryo is at first symmetrical. A larva known as a veliger occurs 
in many forms. 

599. The Gastropods are typically aquatic but there are many forms 
in which the mantle chamber serves as a lung, no gills being developed. 
This is the case with many fresh-water forms and a large number of 
forms which are purely terrestrial. Many Gastropods are vegetable 
feeders. Others are carnivorous, some have the power of boring through 
the shells of other molluscs by means of an acid secretion, and thus killing 
their prey. 

600. The numerous families of Gastropods are classified as follows: 

Legion I. Strep toneura 

Sub-order i. Aspidobranchia 

Sub-order 2. Ctenobranchia 

Sub-order 3. Heteropoda 
Legion II. Euthyneura 

Sub-order i. Opisthobranchia 

Sub-order 2. Pulmonata. 

601. The Streptoneura have the visceral nerves crossed like a figure 8 
because of the twisting of the visceral mass. For the same reason the 
gills lie in front of the heart. The sexes are generally distinct. The 


Aspidobranchia have feather-shaped (double) gills which are free at the 
tips. The Ctenobranchia have a single comb-shaped (single) gill. The 
Heteropoda are pelagic. The foot forms a flat fin. The visceral mass is 
small and the shell poorly developed or wanting. 

602. The Euthyneura have the visceral nerves parallel. They are 
hermaphrodytic. The Opisthobranchia are marine forms with the gills 
usually behind the heart. The Pulmonata are chiefly terrestrial and 
fresh water snails without gills. The mantle cavity serves as a lung. In 
the slugs the shell is greatly reduced or wanting. 

603. Order 2. The Solenoconcha have a horn-shaped shell and body 
and a cylindrical foot. The group is small and the animals are also small. 
They are marine and live in the mud of the bottom. 

FIG. 165. A slug, Limax. (From Galloway, after Binney's Gould.) 

604. Order 3. The Lamellibranchiata are compressed laterally. The 
head is rudimentary. The mantle is large and double, right and left. 
The shell is also double and the two valves are connected by a dorsal 
ligament. The foot is usually wedge shaped. There are two pairs of 
plate-like gills. The animal is usually bilaterally symmetrical but there 
may be considerable deviation from this rule. 

605. The shell is secreted by the mantle and is composed of three 
layers. On the surface is a thin layer of a horny cuticula (periostracum) 
which is formed by the extreme edge of the mantle. The hinge ligament 
is of the same substance but forms a very thick layer. The hinge is elastic 
and causes the shell to gape when the adductor muscles are relaxed. 
Beneath the cuticula there is a thick layer of calcium carbonate deposited 
in a matrix of organic matter (conchiolin). The limey portion of the shell 
consists of two layers, an outer "prismatic" layer and an inner layer of 
"mother of pearl." The prismatic layer is so called because of the colum- 
nar or prismatic arrangement of the substance. The prisms stand perpen- 
dicular to the surface. The "mother of pearl " is in layers parallel to the 


surface. The mantle lines the entire inner surface of the shell. Some- 
times the edges of the mantle are partly united. There are always two 
points, however, where they are not united. One is at the posterior border 
and one is ventral anterior, opposite the foot. The posterior opening is 
frequently double and the edges of the mantle are then often extended 
so as to form a pair of tubes or siphons, or one double siphon. Through 
the ventral siphon the water enters the mantle cavity and escapes by the 
dorsal siphon. 

606. The flattened body is suspended from the dorsal border of the 
mantle lobes. At its anterior and posterior ends are two strong muscles 
which connect the two valves of the shell. These are the adductors which 
close the shell. The gills are suspended from the dorsal border of the body 
in the mantle cavity. Each gill consists of two series of parallel vertical 
bars or filaments which are connected by short longitudinal and trans- 
verse bars. The gill is therefore a sort of double grating. On either 
side of the mouth are two triangular lappets, the labial palps. The 
surface of the gills and palps is covered with cilia which induce the currents 
in the water for respiration and feeding. The minute particles of food 
are carried toward the mouth along the edge of the mantle and thence 
between the palps. The mouth is a simple opening into the short 
oesophagus. The stomach is rather large and receives several ducts 
from the large digestive gland. From the stomach the intestine makes 
a number of loops and passes out of the visceral mass dorsally and pos- 
teriorly to a point above and behind the posterior adductor muscle into 
the mantle cavity. 

607. The heart lies on the dorsal side of the visceral mass and consists 
of a ventricle and two lateral auricles. It is enclosed in a pericardial 
chamber which represents the body cavity. A nephridial funnel opens 
into the pericardial chamber on each side and this connects with the 
kidneys, or organs of Bojanus. The kidneys open into the mantle cavity 
on the side of the visceral mass. The sexes are usually separate. The 
gonads are large paired organs embedded in the visceral mass and opening 
with or near the kidney openings. 

608. The nervous system consists of a cerebral ganglion which lies 
above the oesophagus, a visceral ganglion below the posterior adductor 
muscle, and a pedal ganglion embedded in the foot. These ganglia are 
connected by pairs of nerves. Sometimes there is also a separate pleural 
ganglion. The special sense organs are not well developed. There is a 
double statocyst near the pedal ganglion. Eyes are seldom found on 
the body but in a number of forms they occur on the edge of the mantle 



and on the siphon. Tentacles, or special tactile organs are common on 
the siphon and mantle edge. 

609. Fertilization of the eggs takes place in the mantle cavity where the 
early stages of development also take place in many cases. In the fresh- 
water clams especially, the 

larvae remain for a long time 
attached to the gills of the 
parent. The marine forms 
have a trochophore larva. In 
the fresh-water forms the 
metamorphosis is more com- 
plete and in some cases the 
larvae live for a time as para- 
sites attached to the gills and 
fins of fishes. 

610. All Lamellibranchs are 
aquatic and chiefly marine. 
Most live free on the bottom 
but some are attached by 
byssus threads which are 
formed by the secretion of a 
byssus gland in the small foot. 
Others are attached by the 
cementing of one valve to the 
substratum. Some bore into 
wood and others into calcare- 
ous rocks. 

611. Order 4. The Cepha- 
lopods are the most highly 
organized of all molluscs and 
in some respects of all inver- 
tebrates. All are marine and 
some attain great size. Some 

species are known to attain, a Kingsley.) 

length of 50-60 feet including 

the long arms. The squid, cuttlefish, nautilus, and octopus are some 

of the best-known examples. Except in the pearly nautilus the 

shell is always rudimentary, and completely overgrown by the mantle. 

The visceral mass is elongated, conical in form, and lies in a much 

larger mantle chamber. There are two or four plume-like gills. The 

FIG. 1 66. The "soft shell" clarh, Mya 
arenaria. Showing the position when 
buried in the mud with the siphons extend- 
ing to the surface. (From Galloway after 



mantle is very thick and muscular. The foot is shaped like a funnel and 
projects somewhat beyond the edge of the mantle. By the strong con- 
traction of the mantle a stream of water is shot out through the funnel 
which causes a backward movement of the animal. Less vigorous con- 
tractions of the mantle produce respiratory currents. The large head is 
produced into eight or ten long arms which encircle the mouth. The oral 
surface of the arms is covered with numerous suckers which are purely 
hold-fast organs. The mouth opens into a buccal cavity and is provided 
with two strong jaws which together have the form of a beak. The buccal 
cavity contains a radula, and into it open four large salivary glands. The 

FIG. 167. The Devil-fish (Octopus), a dibranch Cephalopod. A, At rest; B, 
swimming, a, Arms; e, eye; s, siphon. (From Galloway after Merculiano.) 

long oesophagus is sometimes enlarged into a crop. The stomach consists 
of two sacks into one of which two large digestive glands open. A large 
gland secreting ink opens into the rectum near the vent. The vent opens 
into the mantle cavity. The ink is discharged when the animal is pursued 
and serves to cover its flight. The heart lies in the upper side of the 
visceral mass. It consists of a ventricle and as many auricles as there are 
gills, 2 or 4. The arteries entering the gills are enlarged, muscular and 
rhythmically contractile. They are called branchial hearts. There are 
one or two pairs of kidneys intimately connected with the circulatory 
system and also connected with the body cavity by nephridial funnels. 
The kidneys open into the mantle cavity. 


612. The cerebral, visceral, pedal, pleural and buccal ganglia are all 
grouped in the region of the buccal mass. Other ganglia occur at the bases 
of the arms and in other parts of the body. The sense organs consist of 
a pair of eyes, a pair of statocysts and a pair of chemical sense organs below 
the eyes. The eyes are usually very highly developed and have a remark- 
able resemblance to the vertebrate eye, though fundamentally very 

613. An internal skeleton of cartilage supports and protects the eyes 
and central nervous system. Other cartilages are found at the bases of 
the arms, at the edge of the mantle and in the funnel and in the fin. The 
rudimentary shell in most cases serves as a supporting structure. It 
may be either horny or calcareous. 

FIG. 168. The pearly Nautilus, a tetrabranch Cephalopod. e, Eye; h, hood; 
s, siphon; se, septa forming the chambers of the shell; sp, siphuncle; t, tentacles. 
(From Galloway after Nicholson.) 

614. The sexes are separate and dimorphic. The single gonad lies in 
the end of the visceral sack and its products are emptied into the ccelomic 
cavity. A pair of complicated ducts with associated glands lead from the 
body cavity to the mantle cavity. One of these ducts is frequently 
rudimentary or wanting. Development is direct. 

615. The cephalopods are carnivorous, using their arms for catching 
their prey. When on the bottom the arms are also used for locomotion. 
By means of the mechanism already described a strong swimming stroke 
which carries the animal backward is performed. Some species are 
habitually swimming, others keep close to the bottom. 


6 1 6. The sub-order Tetrabranchiata is characterized by the four gills, 
numerous tentacles in place of the arms and a large many-chambered 
shell of which only the last, largest chamber is occupied by the animal. 
The ink bag is wanting. There is only one living species, the pearly 
Nautilus. The Dibranchiata have only two gills and a rudimentary shell. 
In the section Decapoda there are eight arms and two longer tentacle 
arms. The suckers are stalked and have a horny rim. There are two 
lateral fins. In the section Octopoda the tentacles are wanting, the 
suckers are sessile and without a horny rim. There are usually no fins. 

Cephalopoda : 

Sub-order. Tetrabranchiata, Nautilus. 
Sub-order. Dibranchiata. 

Section. Decapoda, Squid. 

Section. Octopoda, Octopus. 

617. PHYLUM IX. Adelochorda. 

6 1 8. Class I. Enteropneusta. This phylum and class are 
represented by a few genera of worm-like animals which are of 
interest because they form one of the links connecting the in- 
vertebrates and vertebrates. A representative of the group 
common on our Atlantic seashore is Dolichoglossus. The 
animal burrows in the sand and mud along shore. When the 
tide is out the coiled castings of this animal are often seen form- 
ing piles several inches in height. The coiled castings and an 
odor of iodoform are indications of Dolichoglossus. The body 
is composed of a conical proboscis, a broad band-like " collar" 
and a long tapering trunk. Only three points in the anatomy 
need be mentioned, i. The mouth lies in front of the collar 
and from here the digestive tract extends directly to the poste- 
rior end of the body. The anterior part of the digestive tract 
is differentiated for respiration. It is connected at regular 
intervals with the body- wall and at these points there are open- 
ings which form a passage from the enteric cavity to the ex- 
terior. These openings are called gill slits. The respiratory 
current enters the mouth and passes out through these slits. 
2. The nervous system consists of two ganglionic chains, one 


dorsal and one ventral. These are connected by a nerve ring 
in the region of the collar. The dorsal nerve chain is tubular 
in front of the nerve ring. 3. The dorsal wall of the digestive 
tract is prolonged forward into the proboscis as a stiff tube of 
cells which forms a supporting axis for the proboscis. Neither 
of these features are found in any of the phyla so far described 
but they are regarded as the homologues of the pharyngeal 
gill slits, dorsal tubular nervous system, and notochord, re- 
spectively, of the Vertebrates. The validity of the third 
homology may be seriously questioned. 

619. PHYLUM X. Urochorda. The Urochorda are also called 
Tunicata because of the tunic or test, a thick integumentary 
structure formed by the mantle in many forms. This test 
is remarkable because it contains cellulose, which is otherwise 
found only in plants. The test is sometimes gelatinous but 
is often extremely tough and resistant. Many Tunicata are 
fixed but there are also free swimming forms. In the adult, 
the animals are usually markedly degenerate. The body is 
often sack-like in form. There is a large pharynx with gill 
slits, a dorsal tubular nervous system and a notochord. The 
food in minute particles is collected from the respiratory current 
and directed to the oesophagus by the action of ciliated grooves 
in the pharynx. The Tunicata are all marine. 

620. Class I. Copelata. This class comprises free swim- 
ming forms in which the notochord persists in the adult. 
The gill slits open directly to the exterior. The body is cask- 
shaped and there is a flat tail. The mantle is readily cast off 
and reformed. 

621. Class II. Tethyodea. The Ascidians or sea squirts 
are for the most part fixed. The gill slits and vent open into 
a chamber, "atrium," formed by folds of the integument. 
The atrial opening is usually near the mouth. Both mouth 
and atrial opening can be closed by muscular contraction. The 
whole body can also be greatly contracted. From the pharyn- 


geal chamber a short oesophagus leads to a stomach into which 
the ducts of a digestive gland open. There is then a short 
coiled intestine which opens into the atrium. There is a heart 
but the vascular system is not well developed. The nervous 
system consists of a single ganglion lying between the mouth 
and atriopore. The sense organs are not well developed. All 
Tunicata are hermaphroditic. The larva develops into a 
"tadpole" which shows marked vertebrate affinities. Extend- 
ing through the tail and for some distance into the body of the 
"tadpole" is a rod of large cells which forms a supporting axis. 
This is the notochord. On its dorsal side is a long tubular 
nervous system which ends in front in a vesicle containing an 
eye and a statocyst. At the anterior end of the body there are 
three glandular papillae by which the tadpole finally attaches 
itself. A metamorphosis then takes place in which the chorda 
entirely disappears and the nervous system is reduced to the 
single ganglion. The entire tail is resorbed and by a twisting 
of the body in the further process of development the mouth 
comes to lie opposite the point of attachment. 

622. Reproduction also takes place asexually and in many 
forms colonies are formed by budding. The colonies may be 
free swimming. 

623. Class III. Thaliacea. The Thaliacea are free swim- 
ming, transparent, colonial forms with an alternation of gen- 
erations. Solitary individuals give rise to a colony of sexual 
individuals by budding and the colonial individuals produce 
eggs which develop into the sexless solitary form. 

624. PHYLUM XI. Acrania. This phylum has so much in 
common with the vertebrates that the absence of a skull was 
considered of sufficient note to be indicated in the name of 
the phylum. The body is elongated, flattened laterally and 
pointed at both ends. There is a persistent notochord, a long 
series of gill slits and a dorsal tubular, nervous system. The 
body is segmented but there are no paired appendages. 


625. Class Leptocardia. There is only one class represented 
by a few species. The form of the body has given rise to 
the common name "lancelet." The integument of the lancelet 
consists of a single layered epidermis. The gill slits open into 
a peribranchial chamber formed by folds of the skin. This 
chamber opens to the exterior by a pore at its posterior end. 
The mouth is surrounded by a circle of cirri. The animal lies 
on its side partly buried in the sand and collects its food from 
the respiratory current which is produced by ciliary action. 
The intestine is a straight tube opening by a vent on the left 
side of the tail. A long glandular pocket connected with the 
intestine probably represents a digestive gland. There is no 
heart but the larger vessels drive the blood by pulsating 
contraction. There is a ventral vessel (truncus arteriosus) which 
carries the blood forward. From this lateral branches pass 
over the gill arches to unite above in another vessel through 
which the blood flows back toward the posterior end of the body. 
A portal vein connects the intestine and digestive gland (liver), 
and other veins from the body-wall and digestive gland unite 
in the truncus arteriosus. In the region of the gills there is a 
paired series of nephridia which begin with funnels in the 
ccelomic cavity and open into the peribranchial chamber. 
The lancelet cannot be said to have a brain. The anterior 
end of the nerve tube (spinal cord) is slightly enlarged and con- 
tains a vesicular enlargement of the central canal. Connected 
with this is a single eyespot and an olfactory groove. Numer- 
ous other eyespots lie scattered along the spinal cord. The 
gonads lie against the wall of the peribranchial chamber which 
breaks to permit the reproductive cells to escape. The develop- 
ment of the lancelet is by a metamorphosis. The early stages 
are described in Part III. 

626. PHYLUM XII. Vertebrata. The Vertebrates all have a 
series of gill slits. In the terrestrial forms these are only 
present in the larval stages. The nervous system arises as a 


tubular infolding of the ectoderm of the dorsal side. The 
axial skeleton consists primarily of a notochord which persists 
in the lower forms but is only found in the larval stages of the 
higher forms. In addition a vertebral column and a skull 
supplement or replace the notochord. The vertebral column 
consists of a series of segmentally arranged vertebrae with dorsal 
arches protecting the nervous system and ventral arches pro- 
tecting the viscera. 

627. Class I. Cyclostomata. The round mouth eels. This 
class comprises only a few species of eel-like animals which 
are destitute of a lower jaw. The skin is smooth, i. e., there 
are no scales. There are no paired appendages. There is a 
median dorsal fin which is continued around the tip of the tail 
forming a tail fin. The mouth forms a circular sucking disc 
which is covered with hard epidermal tubercles by which the 
animal bores through the skin of the host to which it attaches 
itself. The Cyclostomes are ectoparasites and some even make 
their way for some distance into the host. The alimentary 
canal is practically a simple tube, though some forms have a 
spiral valve; and there is a large liver which opens into the 
digestive tract by a duct. There are also glands in the wall 
of the intestine. There are 6-14 pairs of gill slits which open 
directly to the exterior. The skeletal system consists of a 
well-developed notochord with a thick fibrous sheath, and a 
number of cartilages. The brain is enclosed in a skull composed 
partly of cartilage, partly of membrane. To this are attached 
the two cartilaginous ear capsules and a cartilaginous nasal 
capsule. There is also a network of cartilages surrounding and 
supporting the mouth and pharyngeal regions. The vertebrae 
consist only of neural arches with intercalary pieces, and of 
haemal arches in the tail region. There is a heart similar to 
that of fishes. The olfactory organ is a single sack with a 
median opening. The eyes are of the typical vertebrate type 
but in some cases more or less reduced. The ear is a simple 


statocyst-like sack. The brain is well developed, but lacks cere- 
bral hemispheres and cerebellum. The ten cranial nerves are 
of a simple type and the spinal nerves do not unite dorsal and 
ventral roots. 

628. Class II. Pisces. The Fishes are a large group in which 
are included animals of very diverse character. The skin is 

FIG. 169. The Devil-ray, Manta. The lateral expansions are developed 
from the pectoral fins. This is a ventral view and shows the five pairs of gill 
slits, the odd shaped head with the eyes on the sides of the horns, the wide, 
straight mouth, and the whip-like tail. Taken at Beaufort, N. C. Much 

usually covered with bony scales. There is a median fin which 
may be divided into several parts. There may be one or more 
dorsal fins, a tail fin and a ventral fin. There are usually 
two pairs of appendages which also have the form of fins. The 


fins are all dermal expansions supported on cartilaginous rays 
and bony spines. Fishes are all aquatic and respiration is by 
means of gills. There are often accessory respiratory organs 
the swim bladder and true lungs. The swim bladder is an 
unpaired sack filled with air which serves to give the body of 
the fish the same specific gravity as the water. The heart 
consists of three chambers, a thin- walled auricle opening ante- 
riorly into a very strong thick- walled ventricle which in turn is 
continued anteriorly by the conus arteriosus. In the bony 
fishes the latter is wanting and in its place the truncus arteriosus 
is enlarged into a bulbus arteriosus. The vessels have prac- 
tically the same arrangement as in the lancelet. The organs of 
special sense are two nasal chambers which do not communicate 
with the mouth but have each two openings on the surface, one 
incurrent, one excurrent orifice; two eyes; two statocysts with 
utriculus and sacculus and three semicircular canals; and the 
lateral line system. The lateral line organs line depressions or 
tubes which communicate with the surface. These organs are 
arranged in a line along each side of the body and other shorter 
lines along the side and over the dorsal surface of the head. 
The sensory cells are clustered and their sensory bristles 
project into the canal of the lateral line. The function is to 
sense the currents in the water. The cerebral hemispheres are 
small, the cerebellum comparatively well developed. 

629. Order i. The Selachii are the sharks and rays; fishes with a 
cartilaginous skeleton, placoid scales, 5-7 gill clefts with separate openings, 
a spiral valve, and upper jaw not united with the skull. The body is 
spindle shaped in the sharks and flattened dorso-ventrally in the rays. 
The spiral valve is a much enlarged posterior portion of the intestine with 
a spiral shelf-like fold projecting inward from the wall. These fishes are 
chiefly marine. 

630. Order 2. The Holocephali are not numerous in point of genera. 
They^have a cartilaginous skeleton with the upper jaw articulated with 
the skull. The notochord persists and the vertebrae are represented only 


by thin calcareous rings in the chorda membrane. There is only one 
pair of external openings of the gill clefts. 

631. Order 3. The Dipnoi or lung fishes. In this group there are four 
pairs of gills which are covered by an operculum. The tail is diphycercal 
i. e., the tail fin is symmetrical around the straight spinal axis. The 
chorda persists. The skeleton is cartilaginous and partly bony. There 
is a spiral valve and a pair of lungs. These fishes live in tropical regions 
in rivers and ponds which dry up in the dry season. During the dry season 
the animals bury themselves in the mud. 

632. Order 4. The Brachioganoidea also have gills covered by an 
operculum and a diphycercal tail. The skeleton is bony. The body is 
covered with thick rhombic scales covered with ganoin, a kind of enamel. 
There is also a spiral valve. 

633. Order 5. The Chondroganoidea or sturgeons have the gills covered 
by an operculum; a persistent chorda and cartilaginous skeleton; the head 
prolonged into a snout; the skin is naked or with bony plates; the tail 
fin heterocercal, i. e., the axis is bent up and the fin is unsymmetrical; 
a spiral valve and a conus arteriosus. The number of genera is small. 
The fishes are found chiefly in fresh waters. 

634. Order 6. The Rhomb oganoidea or gar pikes have gills covered by 
an operculum; a bony skeleton; a long snout; body covered with rhombic 
ganoid scales; tafl heterocercal; rudimentary spiral valVe and a conus 
arteriosus. A few species only; found in the rivers and lakes of North 

635. Order 7. The Cycloganoidea have operculum, bony skeleton, 
cycloid scales, tail heterocercal, rudimentary spiral valve and a conus 
arteriosus. There is only one species, Amiatus calvus. This is found in 
the streams of North America. 

636. Order 8. The Teleostei or true bony fishes are very numerous. 
They have an operculum, bony skeleton, ctenoid or cycloid scales or 
large bony plates, a bulbus arteriosus, no spiral valve. The order is 
divisible into twelve sub-orders with many families. Some of the families 
are the herrings, salmon, electric eel, carp, catfish, eels, pike, sea horse, 
mullet, perch, mackerel, flat fishes, toad fish, trunk fish, etc. 

637. Class HI. Amphibia. In this group of animals the 
larva is typically aquatic, the adult terrestrial. This is pri- 
marily evident in the respiratory organs though in many cases 
a marked metamorphosis occurs which involves other organs. 
The Amphibia have two pairs of pentadactyl appendages, a 



U-d el. 


FIG. 170. Diagrams of the girdles and appendages of a typical Vertebrate. 
A, Anterior; B, posterior; ac, acetabulum; c, coracoid; ca, carpals; ce, centralia; 
d.c., distal carpals; d.t., distal tarsals; el, elbow-joint; /, fibula; fe, femur; fi, 
fibulare; g.c., glenoid cavity; h, humerus; il, ilium; in, intermediale; is, ischium; 
kn, knee-joint; m.c., metacarpals (1-5); m.t., metatarsals (1-5); p pubis; ph, 
phalanges (1-5); pr.c., pre-coracoid; r, radius; ra, radiale; sc, scapula; t, tibia; 
ta, tarsals; ti, tibiale; u, ulna; ul, ulnare. (From Galloway.) 



naked skin, gills in the larval stages, lungs in the adult, a three- 
chambered heart of one ventricle and two auricles. The larva 
is a tadpole with a broad tail but no paired appendages. It is a 
vegetable feeder and has a long coiled intestine. In meta- 
morphosis the tail. shrivels as the legs develop. At the same time 
the gills are also resorbed and the lungs are developed and be- 

FIG. 171. Salamandra maculosa, the fire salamander of Europe. Slightly 


come functional as respiratory organs. The adult is carnivorous 
and the digestive tract is relatively shorter in correspondence 
to the character of the diet. This is the type of metamorphosis 
which occurs in the frogs and toads. The newts and salaman- 
ders undergo a less radical transformation. The skeleton is 



bony, though parts remain cartilaginous in the adult. The 
skeleton is so much like that of the higher vertebrates that 
most parts can be accurately homologized. The same is true 
of the digestive tract. There is a cloaca into which the intes- 
tine, the ureters and the genital ducts open. On its ventral 
wall there is a large pocket, the urinary bladder. The lungs are 
simple sacks with the walls usually more or less folded like a 
honey comb. The thin skin also acts as a respiratory organ. 

FIG. 172. Outline drawings of three urodele amphibians showing successive 
stages in degeneration of the appendages. A, Siren; B, Amphiuma; C, Necturus. 
(From Galloway, after Mivart.) 

The kidneys open into the cloaca by a pair of ureters. The 
oviducts are two long convoluted tubes beginning in the anterior 
part of the body cavity by large funnel-like openings and lead- 
ing separately into the cloaca. The male gonads are connected 
with the kidneys and the sperm reaches the cloaca by way of the 
urinary tubules. The nostrils have an opening into the anterior 
part of the mouth. The brain is well developed but the cere- 
bellum is small. 

638. Order i. The Gymnophiona are worm-like Amphibia, without 
appendages. The trunk is elongated and the tail rudimentary. The 
skin is filled with small scales. The chorda is persistent. The eyes are 



not well developed and the ear drum and middle ear are wanting. This 
is a small group, found in tropical regions living underground. 

639. Order 2. The Urodela have an elongated body with a tail and 
usually weak legs. In one family (Sirenida?) the posterior legs are 
wanting. In some families the gills are retained in the adult and in 
others the gills are lost but the gill slits are retained. In most cases, how- 
ever, the gills and slit both disappear. The eyes are small and the ear 


FIG. 173. Diagram of a bird embryo within the egg membrane. The foetal 
membranes are omitted (see next figure), b, Brain; b.w., body wall; c.c, central 
canal of spinal cord; co, ccelom; g, intestine; g.w., wall of intestine; s.c., spinal 
cord; y.s., yolk-sac. 

drum and middle ear are absent, To this order belong newts, efts, 
"spring lizards," "mud puppies" and salamanders. 

640. Order 3. The Anura comprise frogs, toads and tree toads. The 
body is short and tailless. The posterior pair of appendages are usually 
long and strong. The eyes are large; there is usually an ear drum with 
a middle ear communicating with the mouth cavity. 

641. Class IV. Reptilia. The Rep tiles a re typically terres- 
trial though many live in the water. They never have gills. 
The skin is covered with horny scales or plates formed by the 



epidermis. There are typically two pairs of appendages but 
these have' been lost in many of the Squamata. The heart has 
two auricles and the ventricle is partly divided by an incom- 
plete partition. The lungs are spongy in structure. There 
are twelve cranial nerves. This comprises the ten cranial 
nerves of the amphibia and the first spinal nerve as well as a 
spinal accessory nerve not represented as a separate nerve in 





- a ;"* 

FIG. 174. Diagram of a bird embryo with the foetal membranes, the amnion 
and the allantois. am 1 , inner or true amnion; aw 2 , outer or false amnion; am.c, 
amniotic cavity; al, allantois. Other lettering as in the preceding figure. 

Amphibia. The cerebral hemispheres are well developed. The 
intestine, ureters and genital ducts open into a cloaca. The 
eggs are fertilized in the oviduct. They are very large and are 
covered by a tough shell secreted by the oviduct. The eggs 
are generally not brooded but are buried in the earth and allowed 
to develop at atmospheric temperature. The embryo is pro- 
vided with the fcetal membranes, the amnion and allantois. 


642. Order i, Rhynchocephalia are represented by a single species 
living on islands off the coast of New Zealand. The animals are lizard- 
like, but more primitive in a number of ways. 

643. Order 2. The Testudinata or turtles have a very compact form 
with bony dorsal and ventral shields, the carapace and plastron. The 
jaws are covered with a horny sheath forming a beak. Teeth are wanting. 
The carapace is formed by the broad dorsal spines and the much ex- 
panded ribs together with a series of marginal plates of dermal bone. 
The plastron is chiefly composed of plates of dermal bone. The shell is 
covered with thick horny scales, the "tortoise shell." Most of the 
Testudinata are aquatic. Snapping turtle, terrapin, tortoise, and sea 

644. Order 3. The Emydosauria are large aquatic lizard-like reptiles. 
The alligator, crocodile and gavial are well known. There are only a 
few genera. The skin contains bony plates as well as horny scales. The 
teeth are set in sockets. The ventricle is completely divided into two 

645. Order 4. The Squdmata comprise both lizards and snakes. 
Usually the lizards have two pairs of appendages while the snakes have 
none, but among the lizards are found various stages of degeneration of 
the appendages even to forms in which no evidence of limbs is discernible 
externally. On the other hand among snakes rudiments of appendages 
are also found. The Squamata are distinguished from the other reptile 
orders by the movable quadrate bone. In the sub-order Lacertilia, the 
lizards, the upper jaws are not movable. The tongue is flat. There is 
a urinary bladder. In the sub-order Ophidia, the upper jaw is movable, 
the tongue is forked and enclosed in a sheath and there is no bladder. 
The ear drum and middle ear are also wanting. Another small sub- 
order of lizard-like forms, including the chameleon, are tree dwellers and 
as a special adaptation to such conditions the toes are opposable for clasp- 
ing, and the tail is prehensile. 

646. Class V. Aves. The Birds are in many respects the 
most highly specialized of all animals. The feathers, which 
are characteristic of the class, are specialized epidermal struc- 
tures and are very remarkable. The skin is comparatively thin 
but the feathers more than compensate as protective structures. 
For resistance to mechanical injury, or protection from cold, 
or heat, or wetting, or adaptation to thermal control, or for the 


possibilities of ornamentation either in form or color, it is diffi- 
cult to find within the entire range of the animal kingdom more 
efficient structures. In their ability to fly we have another evi- 
dence of high specialization. Both pairs of appendages are 
fundamentally pentadactyl but the anterior pair has undergone 
a profound modification. The bones of the upper arm and fore 
arm are of the typical pentadactyl type but the hand is reduced 
to the three matacarpals of the ist, 2nd, and 3rd digits and two, 
three, and one phalanges respectively. The muscular develop- 
ment is concentrated in the muscles which move the wing as a 
whole, viz., those connecting the wing with the sternum. This 
requires a great development of the surface of the sternum and 
its keel to which these muscles are attached. The other mus- 
cles of the wing are greatly reduced. Another anatomical 
peculiarity which is thought to be an adaptation to flight is 
the comparatively small head of the bird. This is due chiefly 
to the absence of the organs for mastication, teeth, heavy upper 
and lower jaws and heavy masseter muscles. The absence of 
these organs is compensated for by the crop in which the food 
is softened, and the gizzard in which it is triturated. By this 
substitution the weight of the body is concentrated and the 
centre of gravity lowered. The posterior appendages are also 
peculiar. The pelvic girdle is attached to at least six vertebrae 
but is open below, that is, there is no symphisis pubis. This 
condition of the pelvic girdle allows of the passage of the rela- 
tively very large eggs of the bird. The fibula is rudimentary 
and the proximal tarsals are fused with the tibia forming a 
single bone, the tibio-tarsus. The distal tarsals are fused with 
the metatarsals to form a tarso-metatarsal. The fifth, and 
sometimes the first, digits are wanting. 

647. The heart is completely divided into four chambers as in 
the Crocodilia and Mammals but the blood from the left ventricle 
goes to the lungs while that from the right goes to the general 
systemic circulation, reversing the order as found in Mammals. 

AVES 299 

The lungs are connected with an extensive system of air spaces 
which penetrate far into other parts of the body, even penetra- 
ting the bones and replacing the marrow. The vocal cords 
are located at the junction of the bronchi in an organ, the syrinx, 
which takes the place of the larynx. The right ovary is want- 
ing and the corresponding oviduct is rudimentary. The eggs 
are fertilized in the upper part of the oviduct and are then 
surrounded by layers of albumen, membraneous shell and cal- 
careous shell in succession, as they pass down the oviduct. 

648. The eyes and ears are highly developed. There are two 
eyelids and a nictitating membrane. The ear is without a 
concha. The brain shows a considerable advance over that of 
Reptiles, especially with regard to the development of cerebrum, 
optic lobes and cerebellum. 

649. In many points Birds differ radically from Mammals 
and at the same time show a strong resemblance to Reptiles. 

650. In a comparatively small group of birds the wings are not used 
and are consequently rudimentary. These are the running birds or 
Ratitae ostrich, emeu and cassowary, the almost extinct apteryx of New 
Zealand and the recently extinct moa of New Zealand. With the disuse 
of the wing the muscles have, degenerated and with them the keel of the 
breast bone, their point of attachment. Other birds are called Carinatae 
because of the keel of the breast bone. They are divided into seventeen 
orders as indicated in the following synopsis. 


1. Struthiomorphce. Ostrich, rhea, cassowary. 

2. Dinornithes. The recently extinct gigantic Dinornis. 

3. Aepyornithes. The recently extinct yEpyornis. 

4. Apteryges. The Kiwi Kiwis of Australia and New Zealand. 

5. Tinamiformes. South American fowl-like birds. 

6. Gallinacei. Pheasants, turkey, fowl, quail. 

7. Columba. Pigeons, doves. The extinct dodo. 

8. Lari. Gulls. 

9. GrallcE. Rails, cranes. 

10. Lamellirosires. Geese, ducks, flamingo. 

11. Ciconice. Ibis, storks, herons. 



12. Steganopodes. Pelican, frigate bird. 

13. Tubinqres, Stormy petrel, albatross. 

14. Impennes. Penguin. 

FIG. 175. The kiwi, Apteryx. Xi/4- The outline in the background 
represents the size of the wing. 

15. Pygopodes. Divers, grebes. 

1 6. Accipitres. Condor, vultures, eagles, hawks, falcons. 

17. Striges. Owls. 

AVES 301 

1 8. Psittad. Parrots. 

19. Coccygomorpha. Cuckoo. 

20. Pid. Woodpeckers. 

21. Cypselomorpha. Whip-poor-will, bull bat, humming birds. 

22. Passer es. The song birds; a very large order. 

Sub-order i. Clamatores. King bird. 

Sub-order 2. Oscines. Swallows, fly catchers, warblers, thrushes, 
black birds, mocking bird, cat bird, larks, titmouse, crows, ravens, 
finches, sparrows. 

651. Class VI. Mammalia. The Mammals are typically 
covered with hair. In several cases the hairs are scattered or 
limited to the " whiskers" as, e. g., in some marine mammals 
the sea cow and whale. The function of the hair is primarily 
to retain the heat of the body. When the hair is wanting the 
function may be performed by a thick layer of fat beneath the 
skin. The name mammal refers to the mammary glands in 
the skin of the ventral surface of the body. There are usually 
two sets of teeth, first a milk dentition which is later replaced 
by a permannt dentition. The latter consists of four kinds, 
incisors, canines, pre-molars and molars. The heart consists 
of four chambers; the red blood corpuscles are without a 
nucleus; the temperature of the body is constant. The lungs 
and heart are separated from the abdominal viscera by a muscu- 
lar membrane, the diaphragm, which thus divides the body 
cavity into thoracic and abdominal cavities. There is no 
cloaca, the vent and the openings of the urino-genital systems 
are separate. The eggs are fertilized in the oviduct and develop- 
ment continues for a variable period, up to two years in the 
elephant, in an enlargement of the oviduct called the uterus. 
The embryo is provided with the fcetal membranes, amnion 
and allantois. The sense organs are usually highly developed 
and the brain, especially the cerebral and cerebellar parts, is 
much in advance of those of all other animals. 

652. The thirteen orders belong to three sub-classes as follows: 
Sub-class Monotremata. Spiny ant-eater, duck mole. 



Sub-class Marsupialia. 

Order i . Polyprotodontia (carnivorous or omnivorous) . 

Order 2. Diprotodontia (herbivorous). Kangaroo. 
Sub-class Monodelphia. 

Order i. Insectivora. Moles, shrews, hedgehog. 

Order 2. Chiroptera. Bats. 


FIG. 176. The spiny ant-eater, Tachyglossus. Xi/4- 

FIG. 177. The duck-bill, Ornithorhynchus. Xi/4. 

Order 3. Rodentia. Rats, mice, rabbits, squirrels, beavers. 
Order 4. Edentata Nomarthra. Scaly ant-eater. 
Order 5. Edentata Xenarthra. Sloth, armadillo. 
Order 6. Carnivora, Dogs, bears, cats, seals. 
Order 7. Cetacea. Whales, porpoises, dolphins. 


Order 8. Ungulata. Hoofed animals. 
Order 9. Sirenia. Manatee and dugong. 
Order 10. Primates. Monkeys, apes, man. 

653. The Monotremata are the most primitive mammals. The embryo 
is not nourished in a uterus. An egg is laid from which the embryo hatches 
in a very immature condition. It is then nourished from the mammary 
glands. The long snout is covered by a horny sheath like a duck's bill. 
There are no teeth in the adult. The mammary glands have no nipple. 
Many reptilian characters are presented as, e. g., in the presence of a 
coracoid bone, a cloaca, a variable body temperature, the slightly devel- 
oped corpus callosum, the condition of the reproductive organs, etc. 
The Monotremes are found only in Tasmania, Australia and New 

654. The Marsupialia, with exception of the American opossum, are 
also confined to Australasia. There is no placenta and the young are 
born in a very immature stage. They are then placed in a sack formed 
by a fold of the skin covering the region of the mammary glands. Here 
the young attach themselves to a nipple and continue their development. 
The pouch is supported by two bones. 

655. The Polyprotodontia are carnivorous or omnivorous Marsupials. 
They have a well-developed set of teeth of the four kinds. 

656. The Diprotodontia are vegetable feeders and the teeth are not 
developed as a full set. 

657. The Monodelphia have no pouch. The fcetal membranes form a 
placenta which becomes attached to the wall of the uterus thus forming 
an organic connection between the embryo and the tissues of the mother. 
By means of the placenta the embryo is nourished for a period within the 
uterus. After birth it is nourished from the milk of the mammary glands. 

658. The Insectivora have a full set of pointed teeth. The feet usually 
are five toed and the toes clawed. The foot is plantigrade. 

659. The Chiroptera have wings formed by a membrane of skin stretch- 
ing between the greatly elongated fingers and the side of the body. In 
some cases the eyes are large and the ears small, in others the eyes are 
small and the ears large. Some are fruit eaters, others catch insects. A 
few are blood sucking. 

660. The Rodentia are characterized by the long, sharp, chisel-shaped 
incisors. There are no canines. Rabbits, squirrels, beaver, pocket 
gopher, mice, rats, guinea pig. 

66 1. The Edentata are either without teeth or have poorly developed 
teeth without enamel. Scaly ant-eater, armadillo, ant bear. 


662. The Carnivora are flesh-eating mammals with a characteristic 
dentition. The clavicle is rudimentary or wanting. The toes are clawed. 

663. Sub-order Fissipedia. Terrestrial carnivores with molar teeth 
unlike. Canidae; dogs, wolves (digitigrade). Ursidae; bears, (planti- 
grade). Procyonidae; raccoon, (plantigrade). Mustelidae; weasel, ferret, 
mink, pole cat. Viverridae; civet cat (ichneumon). Hyasnidae; hyaena. 
Felidae; cats, lions, tigers, leopards, panther. 

664. Sub-order Pinnipedia. Aquatic carnivores with webbed feet and 
molar teeth alike. Anterior and posterior appendages well developed. 
Otters, walrus, seals. 

665. The Cetacea are aquatic mammals with anterior appendages in 
form of paddles, the posterior ones only represented by internal rudiments. 
There is a broad horizontal tail fin. In the baleen whale the teeth are 
wanting; a curtain of fringed horny plates suspended from the roof of the 
mouth acts as a strainer to collect the food. Whales, narwhal, dolphins. 

666. The Ungulata have broad toes covered with a horny hoof. Sub- 
order Proboscidia; elephants, 5 toes, thick skin, long proboscis. Sub- 
order Perissodactyla ; number of toes odd. Tapir, 4 toes on anterior 
appendages, 3 on the posterior. Rhinoceros, 3 toes. Horse, i toe. Sub- 
order Artiodactyla, even number of toes. Section i. Non-ruminants: 
Hippopotamus, 4 toes; swine, 2 long, 2 short toes. Section 2. Ruminants: 
Tribe i. Tylopoda; camel, dromedary, llama, toes 2, no horns or antlers. 
Tribe 2. Traguloidea; toes 2 long, 2 short (small, hornless, deer-like 
animals of West Africa). Tribe 3. Pecora; usually with horns or antlers, 
toes 2 long, 2 rudimentary. Deer family with antlers. Cattle family 
with horns; antilope, buffalo, cattle, goats, sheep. Giraffe family with 
two toes, horns covered with skin. 

Order Ungulata: 

Sub-order i. Condylarthea. Phenacodus, extinct. 
Sub-order 2. Hyracoidea. Hyrax the coneys of Africa and Ara- 

Sub-order 3. Proboscidea. Elephants. 

Sub-order 4. Perissodactyla (odd-toed). Tapirs, rhinoceros, horse. 
Sub-order 5. Artiodactyla (even-toed). 

Section i. Non-ruminantia. Hippopotamus, swine. 
Section 2. Ruminantia: 
Tribe i. Tylopoda. 
Tribe 2. Traguloidea. 
Tribe 3. Pecora. 


667. The Sirenia are herbivorous marine mammals. The anterior 
appendages are paddle like, the posterior rudimentary. Manatee, dugong, 
sea cow. 

668. The Primates have a heterodont dentition, all appendages have 
five digits. The nails are flat. The eyes are directed forward. Sub- 
order Prosimiae; the teeth similar to those of Insectivores, the wall of the 
orbit incomplete laterally. Lemurs. Sub-order Simiae; the incisors are 
chisel shaped, the orbit wall complete. Monkeys and apes. 

Section i. Platyrhina; the flat nose monkeys of South America. 
Section 2. Catarrhina; the monkeys, mandrels, gibbons, orang- 
utan, chimpanzie and gorilla. 

669. Finally the genus Homo is placed by some authors in an order by 
itself, the order Bimana. Others place this genus in a family, Hominidae, 
under the order Primates. The genus is regarded as containing only one 
living species, Homo sapiens, which is sub-divided into races. 



670. Spontaneous Generation. It was at one time held that 
some animals originate spontaneously. In the middle of the 
seventeenth century the great anatomist, Harvey, expressed 
the view that all living things spring from eggs (Omne vivum 
ex ovo). But this opinion was not generally accepted. A 
quarter of a century later another anatomist, Redi, showed that 
the maggots which develop in decaying flesh are bred from the 
eggs deposited by flies. But for two centuries more spon- 
taneous generation was thought to account for the appearance 
of many living things, though it came gradually to be limited 
to the microscopic organisms, like the bacteria and protozoa. 
Finally, in the latter half of the nineteenth century the experi- 
ments of Pasteur and others definitely established the view that 
even for these microscopic forms a living germ is necessary to 
development of a living organism. It was shown that if the 
germs of the organisms which produce fermentation and decay 
were carefully excluded from the substances in which they 
usually occur, that the processes of fermentation and decay 
would not take place and the associated organisms would not 

671. Continuity of the Living Substance. At the present 
time the term egg is applied only to special cells produced by 
multicellular organisms, from which new individuals develop, 
but the unicellular organisms produce spores which are, in 
this sense, the counterpart of eggs. Another objection to 
Harvey's phrase may be made on the ground that new individ- 
uals may be produced by methods, such as budding, fission, 



etc., in which neither eggs nor spores occur. Still, in essence, 
it is now generally accepted as true that all living things 
originate from eggs, and in this statement is expressed one of 
the most remarkable attributes of the living substance, that of 
its continuity. 

FIG. 178. Amoeba vespertilio, showing the structure of the protoplasm. The 
outer layer is denser and more homogeneous, and is called ectoplasm. The 
central part, called endoplasm, has the appearance of foam. (From Marshall 
after Doflein.) 

672. Structure of Protoplasm. Much has been learned con- 
cerning the physical and chemical properties of protoplasm, 
but even our best microscopes and most refined chemical 
methods still leave much more to be determined. As seen 
through the microscope the cytoplasm seems to consist of: (i) 
a ground substance of a transparent, colorless, homogeneous 



fluid, and (2) a network of a more viscid and more highly 
refractive and sometimes granular substance. This network 
may be either the sectional view of a sponge-like structure or 
of an emulsiform structure or of true fibres interlaced. The 
granules, which are called microsomes, vary greatly in size and 
number and are frequently absent or too small to be seen. They 
are often less conspicuous in the peripheral layers of the cyto- 
plasm, which is therefore distinguished as ectoplasm. 

(Attraction-sphere enclosing two centrosomes.) 

Plastids lying 
in the cyto- 


Passive bodies 
suspended in 
the cyto- 
plasmic mesh- 

FIG. 179. Diagram of a cell. (From Hegner's Zoology, after Wilson, published 
by the Macmillan Co.) 

Beside these constant constituent elements of the cytoplasm 
there are also a large number of structures, which occur only 
in certain cells or at certain times. Among these may be men- 
tioned here the chromoplasts and amyloplasts, the vacuoles 
filled with cell sap, the ingested food particles, the reserve elabor- 
ated food substances, such as starch, oil, aleurone, and the se- 


cretions and other like substances resulting from the activity of 
the protoplasm. With the last named class of cell constituents 
may be included the cell membrane or cell wall. This is usually 
present in 'plants and usually absent in animal tissue, though 
many exceptions occur to both rules. 

673. The Nucleus. The nucleus must be regarded as an 
essential part of the cell. It is true there are certain lowly 
organisms, such as the bacteria, blue green algae and related 
forms, in which there is no such distinct, highly complex 
structure as the typical nucleus; but even in these forms there 
are found scattered in the protoplasm of the cell minute bodies 
which have properties recognized as belonging to constituents 
of the nucleus. These are generally regarded as representing 
the nucleus. 

674. The typical nucleus is a round or oval body, but it may 
also be greatly elongated or even branched. It is usually single, 
but sometimes it consists of two parts, a large macronucleus 
and a small micronucleus. Sometimes there are several or 
even many nuclei in one cell. Usually the nucleus is provided 
with a membrane, but this disappears at certain times, and in 
some cases is entirely absent. Like the rest of the protoplasm, 
the nucleus is transparent and colorless, and in the living condi- 
tion appears homogeneous. But if the cell is treated with cer- 
tain "fixing" and staining reagents, the nucleus becomes 
deeply colored, due to the affinity of one of its constituents for 
the dye. Because of its tendency to stain, this substance is 
called chromatin. In the "resting" nucleus the chromatin 
assumes a great variety of forms; sometimes it is in the form of 
granules of various sizes, more often it is best described as a 
mass of knotted and tangled threads. The chromatin is ap- 
parently a very important constituent of the cell, and in it 
centre many most interesting phenomena. Another element 
of the nucleus which may be stained is the nucleolus. There 
is often only one, but there may be more. They are usu- 


ally spherical masses and, therefore, distinguishable from the 
chromatin. But a better means of distinguishing between 
these is given by the fact that they are not stained by the 
same dyes. 

675. The linin is a part of the nucleus which does not stain 
at all by ordinary methods. It also assumes various forms, 
but when most evident it is as a system of fibres, or a network 
of threads by which the other elements of the nucleus are 
bound together. 

676. The interstices of the nucleus are filled with a nuclear 

677. There is one other structure in the cell which must be 
mentioned, though there is some doubt whether it should be 
classed with the cytoplasmic or nucleoplasmic structures. This 
is the centrosome. It is generally found in the cytoplasm close 
by the side of the nucleus, but sometimes it is far removed, and 
again it seems to be enclosed by the nuclear membrane. The 
centrosome is excessively small, scarcely more than a point, 
even with the highest powers of the microscope, but it may be 
stained by certain methods, and is further distinguished from 
other minute protoplasmic structures by the radial arrange- 
ment of the surrounding protoplasm, for which it forms a 
centre. Something concerning its significance will appear in 
the discussion of cell division. 

678. In this brief description of the protoplasm, only the 
most important constant structures have been mentioned. 
These have each their optical, physical, and chemical peculi- 
arities. The list of substances which have been recognized 
might be greatly extended and yet, because of our imperfect 
instruments and methods we are far from having made a com- 
plete analysis of the protoplasm. It seems probable that fur- 
ther investigation will show that among the innumerable mi- 
nute particles which at present are indistinguishable one from 
another are many chemically and otherwise distinguishable kinds. 


679. Chemical Structure of Protoplasm. Protoplasm con- 
tains a very large percentage of water, 70 per cent or more; it 
is alkaline in reaction in the living condition and contains many 
mineral salts, which, however, vary greatly with the kind of 
protoplasm. Among the chemical elements which may be 
found are phosphorous, manganese, magnesium, calcium, 
sodium, chlorine, and iron. It does not necessarily follow 
that these substances form an integral part of the protoplasmic 
molecule. They may be present as inorganic salt dissolved in 
the cell sap or even in crystalline form. Chemically, the living 
substance is classed with the albumens, but it were perhaps 
better to say that on analysis it decomposes into a series of albu- 
minous compounds. These are themselves extremely complex 
organic bodies, and as yet lie somewhat beyond the range of the 
chemist's power of analysis. An analysis of egg albumen 
yielded the result, C72Hio6Ni 8 SO 2 2, though this cannot be re- 
garded as a correct chemical formula. Nucleoplasm is dis- 
tinguished from the cytoplasm by the presence of phosphorous. 
From all that we know regarding protoplasm we are led to re- 
gard it as an aggregate of many highly complex organic 

680. Function of Cytoplasm and Nucleus. Much light has 
been thrown on the question of the function of cytoplasm and 
nucleus by a series of simple experiments. If a unicellular 
organism is cut into two parts, so that the nucleus is also 
divided, the wound immediately " heals," and each half 
regenerates the part cut away so that eventually there are two 
complete organisms. The operation does not itself greatly 
injure the cell. If the cell is divided so that all of the nucleus 
remains in one part, the part without nuclear matter, even 
though it is the larger part, does not regenerate. It may 
remain alive for weeks, but ultimately dies. If the nucleus is 
completely removed from the cytoplasm, both nucleus and 
cytoplasmic parts die. The death of enucleated portions is 


evidently due to the lack of nutrition. The cytoplasm retains 
its irritability and responds to stimuli; the pseudopodia may 
still be formed or the cilia continues to move, as the case 
may be, but food particles are no longer ingested, and those 
contained in the food vacuoles are no longer digested. The 
cytoplasm seems to have lost the function of assimilation and 
consequently starves. 

68 1. These experiments indicate that the animal functions, 
irritability and contractility, are functions of the cytoplasm. 

682. On the other hand, from what has just been said, it is 
seen that the nucleus has to do with the function of assimilation. 
This is further evident in many cases in which cells are especially 
active in the absorption of food. In such cases the nucleus is 
prolonged into curious finger-like processes on the side of special 
activity. In other cases the nucleus shows evidence of special 
activity where secretory processes are prominent. Here its 
surface also projects toward the point of activity. Since assimi- 
lation and secretion are two phases of metabolism it is natural 
that both should be controlled from the same source, and that 
the nucleus should present similar appearances in both cases. 
In addition to the control of metabolism the nucleus also has 
the function of cell division or reproduction.. 

683. Cell Division. The process of cell division is such a 
complicated one, and with it are connected so many important 
biological phenomena that it demands careful study. The first 
evidence of preparation for cell division is seen in the rearrange- 
ment of the chromatin. This gradually assumes a more regular 
form. The irregular clumps and strands take on the form of 
one or more coiled bands. These have at first an irregular out- 
line, which gradually becomes smoother. The bands become 
thicker and shorter and finally are seen to consist of a limited 
and definite number of V-shaped bodies, to which the name 
chromosomes has been given. During these changes of form 
the affinity of the chromatin for the stains increases. At about 



this time two centrosomes make their appearance close beside 
the nucleus. They are connected by a number of slender fibres, 
which curve outward in the middle so that all together form 
a spindle-shaped figure. The centrosomes gradually recede 

FIG. 180. Diagrams illustrating the prophases of mitosis. A, Beginning of 
formation of the spindle. B, chromosomes formed. C, Chromosomes approach- 
ing the equator of the spindle. D, Chromosomes ready to divide. (See next 

from each other until they come to lie at opposite poles of the 
nucleus. During these changes the nucleoli have gradually dis- 
appeared and the nuclear membrane also becomes indistinct 
and fades away. The spindle fibres now traverse the nucleus 



and some are attached to the chromosomes. Other fibres ex- 
tend from the centrosomes outward into the cytoplasm, and 
the constituents of the cytoplasm take on a radial arrangement 
with the centrosomes as centres. The chromosomes lie regu- 
larly arranged around the spindle in its equatorial plane. 

FIG. 181. Diagrams illustrating the metaphase and anaphases of mitosis. 
A, Chromosomes divided; B, chromosomes approaching the centrosomes; C, 
cytoplasm beginning to divide, the centrosomes also divided; D, cell completely 
divided and nuclei in resting condition. (This and preceding figure from 
McMurrich, adapted from E. B. Wilson.) 

684. The process of division begins with the splitting of each 
chromosome lengthwise into two equal and similar parts, 
whereby the number of chromosomes is doubled. The two 
halves of each original chromosome separate and move in 


opposite directions, each one approaching one of the centro- 
somes. In this way two groups of chromosomes of equal 
numbers are formed at opposite ends of the spindle. At about 
this time a groove appears in the surface of the cytoplasm in 
the equatorial plane of the spindle. This groove cuts deeper 
into the cell until it is divided into two equal masses. Thus the 
nucleus and cytoplasm are divided. 

685. The process of division is concluded by the formation of 
a nuclear membrane around each group of chromosomes and the 
rearrangement of the chromatin. The chromosomes lose their 
individuality again in a tangle of chromatin and the nucleoli 
reappear. The spindle fibres and attraction sphere disappear 
and the centrosomes may also be lost among the other granules 
of the cytoplasm. 

686. This process of cell division is called mitosis or karyo- 
kinesis. It is the normal method of cell division, but under 
certain conditions a simpler process occurs. This has been 
described elsewhere. With slight modification the description 
just given of the mitotic method will apply generally to both 
animals and plants. Several additional points may be 

687. Number of Chormosomes. The number of chromo- 
somes is constant for any given species. For different species 
the numbers observed vary from two in the Nematode, Ascaris 
megalocephala, to 168 in Artemia, a genus of Crustacea. The 
most common numbers recorded are 12, 16 and 24. 

688. Nucleoli. The fate of the nucleoli during mitosis is 
in question. There is some reason for believing that in some 
cases at least they take part in the formation of the chromo- 
somes. More often they seem to disintegrate, and then new 
ones are formed on the organization of the new nucleus. 

689. Centrosomes. The centrosomes are apparently per- 
manent cell structures which propagate themselves by divi- 
sion. At the close of cell division the centrosome of e< 


daughter cell often divides before it is lost in the granular pro- 
toplasm of the resting cell. In many cases the centrosome or 
centrosomes can be found in the resting cells. In other cases, 
division of the centrosome occurs just previous to cell division. 

690. Spindle Fibres. The spindle fibres are achromatic sub- 
stances, i. e., they do not stain like the chroma tin. They are 
probably derived in large part from the linin network of the 
nucleus. The astral figures and, perhaps, the spindle in part, 
are cytoplasmic. 

691. Resting Nucleus. Ordinarily, after division, the cells 
remain quiescent for a time; that is, so far as any apparent 
changes in the nucleus are concerned. This is called the 
resting state, although growth and other processes may be 
actively going OP. 

692. Conjugation. Another very important process in 
which the nucleus is especially concerned is the reverse of cell 
division. Cell division is ordinarily followed by a period of 
growth, then another cell division and another period of growth. 
This constitutes the ordinary daily routine of cell life. At 
comparatively long intervals this chain of events is broken by the 
fusion of one cell with another. This may take place in a great 
variety of ways, which, however, are apparently fundamentally 
the same, and the significance of the process may best be 
explained by the description of a number of examples. 

693. Mougeota is a filamentous alga, which consists of a 
series of similar cells adhering to each other in a row. At 
certain times two such filaments, lying side by side, become con- 
nected by a series of tubes in such a way that each cell of one 
filament is connected with a similar cell in the other filament. 
Then the protoplasm of each cell flows into the tube where the 
two masses unite into a single body. In some other algae swarm 
spores are formed and set free in the water. Each 
spore is pear-shaped and has two flagella by which it actively 
swims about. They are all of the same size and cannot be 


distinguished one from the other. These swarm spores unite in 
pairs, and from each pair a single protoplasmic mass is formed, 
which is, in fact, a single cell. Spirogyra is another filamentous 
alga similar to Mougeota, but when two filaments unite by 
tubes the protoplasts of one flow completely across to the other, 
where the union takes place; the protoplasts of the second cell 
remaining passive. In another example of swarm spores, two 
kinds of spores are produced. The difference is not great, but 

FIG. 182. Conjugation and differentiation of sex. A, Conjugation in Mou- 
geota; B, conjugation in Spirogyra; C, conjugation in Hydrodictyon (isogamous) ; 
D, conjugation in Chlamydomonas (heterogamous). In C: i, a gamete; 2, con- 
jugation; 3, zygote. In D: i, a male gamete; 2, female gamete. 

one is somewhat larger than the other. Here the union takes 
place between individuals of different kind. In Fucus, a 
brown sea weed, the two kinds of cells differ greatly in size. 
The large one is motionless, while the small one has two 
flagella and is very active. In all the higher plants and 
animals the difference in size is enormous, and in animals espe- 
cially, the smaller motile cell consists of little more than 
a small and compact nucleus with a single flagellum. 

694. Fertilization. In the lower forms, where the two unit- 
ing cells are equal in size or nearly so, the process in question is 


called conjugation; in those instances where there is a great 
difference in size, it is called fertilization. The conjugating 
elements are called gametes. In case of fertilization the large 
cell is called a macrogamete or egg, while the small one is called 
a microgamete or sperm. Further, the individual giving origin 
to a macrogamete is called female, and the one from which the 
microgamete springs is a male. A further comparison between 
male and female will be made a little later. 

695. Maturation. In every case of conjugation or fertili- 
zation the nuclei of the two cells sooner or later unite, and the 
real significance of the process centres in the nuclei. The state- 
ment has already been made that the number of chromosomes 
is fixed for a given species, a condition that is maintained by the 
splitting of the chromosomes at each cell division. On the 
fusion of the nuclei, however, the number would be doubled 
were it not for the preliminary process by which both nuclei 
are prepared for the approaching'fusion. To explain this proc- 
ess we will take as an example the sperm cells of Ascaris megalo- 
cephala. The typical number of chromosomes in this species 
is four, and this is also the number in the cell divisions which 
lead up to the formation of the sperm mother cells. The latter 
remain for an unusually long period in the growing, resting 
condition and attain unusually large size. When mitosis begins 
it is seen that the four chromosomes have already split into 
eight which, however, still remain paired. The four pairs ar- 
arnge themselves in two groups of four (tetrads) in the equator of 
the spindle, and the cell divides into two halves, after the usual 
method. Then the centrosomes immediately divide and form 
new spindles, so that a second division occurs before the nucleus 
has entered the resting condition. In this second division the 
four daughter chromosomes do not split, but one chromosome 
from each of the original two tetrads moves toward each pole of 
the spindle. There are thus formed four similar cells, each 
with two chromosomes. These are the sperm cells. 



696. Ill the development of the egg the process is similar to 
that of the sperm up to the period of growth. The mother cell 
of the egg continues to grow for a long time, so that a cell of 
comparatively gigantic proportions results. When the nucleus 
proceeds to divide, four pairs of chromosomes appear, as in the 

FIG. 183. Diagram illustrating the reduction of the chromosomes during 
spermatogenesis. (McMurrich.) sc l , Spermatocyte of the first order; sc 2 , 
spermatocyte of the second order; sp, spermatid. The number of the chromo- 
somes is supposed to be 8( = 2x) in the zygote, and 4( = x) in the gametes. 

sperm mother cell, and they are likewise arranged in two groups 
of four. The nucleus, however, comes to the surface of the cell 
and the spindle takes a radial position, so that on division of 
the cell there results one very small cell (first polar body) and 
one very large one. Then, as in the case of the sperm, the 



nucleus of the large cell divides again without an intervening 
resting stage, and a second polar body is formed. As a result 
of these two divisions there are now one large cell the ripe 

FIG. 184. Diagram illustrating the reduction of the chromosomes during the 
maturation of the ovum. (McMurrich.) o, Ovum; oc l , oocyte of the first 
generation; oc 2 , oocyte of the second generation; p, polar globule. The number 
of -the chromosomes is supposed to be 8( = 2x) in the zygote, and 4( = x) in the 

egg and two small ones. The nucleus of the egg cell has two 
chromosomes, and so has the second polar body. If the first 
polar body were to divide in the same way, there would have 
been three polar bodies, each with two chromosomes. This 



actually takes place in many animals, but the polar bodies 
undergo no further development and are to be regarded as 
parts ejected from the maturing egg cell as useless. It is also 
reasonable to regard the first polar body of Ascaris as poten- 
tially equivalent to two second polar bodies. This leads then 
to the conclusion that the egg nucleus and the nuclei of the 
polar bodies are the equivalents of the four sperm nuclei which 
developed from one sperm mother cell. 

697. The essential difference between the ripe egg cell and 
the sperm lies in the great size of the egg and the motility of the 

Oocyte I ( ) ( ) cyte I 



O O O O O O Ospermatids 

Polar globules 

FIG. 185. Diagram to show the similarity in the development of ova and sperm 
cells. (McMurrich.) 

sperm. The great size of the egg is due in part to the prolonged 
growth period, during which the quantity of protoplasm is 
greatly increased and reserve food in the form of yolk granules 
is stored up within the cell, and in part to the formation of two 
(three) rudimentary cells (polar bodies) in the process of matura- 
tion instead of four equals cell, which leaves one with the devel- 
opmental material which would otherwise be divided among 
the four. These are evidently provisions for the early develop- 
mental period of the embryo. 

698. The consequences of these preparatory processes are 
now readily seen. The second maturation division is called 


the reduction division, because it leaves the nucleus with half 
the normal number of chromosomes. When now fertilization 
takes place the sperm nucleus and the egg pro-nucleus fuse, and 
there results therefrom a new egg nucleus with the normal num- 
ber of chromosomes. In many ways minor variations occur, 
but the essential features seem to hold throughout the animal 
and vegetable kingdoms. Only one variation will be described. 

699. Conjugation in Potozoa. In many protozoa conjuga- 
tion occurs in a peculiar form. The cells do not fuse and only 
the nucleus is greatly affected by the process. In Paramcecium, 
for example, two individuals adhere to each other by their oral 
surfaces and remain in this condition for some time. Finally, 
they separate, the same two individuals, at least in external ap- 
pearance. However, radical changes have occurred in the nuclei, 
which are briefly as follows: The macronucleus disintegrates 
and finally disappears without apparently having taken any 
part in the process of conjugation. The micronucleus forms a 
spindle and divides. This is repeated so that four daughter 
nuclei are produced. Three of these also disintegrate while the 
fourth divides again. There are now two active nuclei in each 
cell, and one of these passes out of one cell to the other through 
the cell mouths, which are placed one over the other. These 
nuclei are regarded as the equivalents of sperm nucleus and egg 
pro-nucleus, and each is said to have approximately half the 
usual chromatic substance. From the fusion of the two nuclei, 
the receptive nucleus and the wandering nucleus, a new nucleus 
is produced, from which the new macronucleus and the new 
micronucleus are both developed. 

700. Fertilization. The approach of the sperm to the egg 
cells is not a matter of accident. Observation of the movement 
of the active sperm in the presence of a ripe egg is sufficient to 
persuade one that there is a positive stimulus which directs them 
to the egg. In a short time large numbers are swarming about 
the egg and apparently endeavoring to penetrate its surface. 













FIG. 1 86. Diagram of the process of conjugation in Paramcecium (on the 
left), and of maturation and fertilization (on the right). In A and / the out- 
lines of the cells are represented and the condition of the nuclei before and after 
the changes they undergo. In each case white and black are used to distinguish 
between the nuclear material of the two parent cells. In B-H the cell out- 
lines are omitted. 

In Paramoeceum (on the left); A and B, temporary union of the two cells; C, 


By experiment it can be shown that the sperms of mosses are 
attracted by solutions of cane sugar and fern sperms are at- 
tracted by solutions of malic acid. These or similar sub- 
stances are probably excreted by the egg and serve as a direct- 
ing stimulus to the sperm. 

701. Many eggs are enclosed in a gelatinous envelope, which 
is readily penetrated by the sperm. In eggs which have a firmer 
covering, or "shell," there are one or more pores (micropyles) 
through which the sperms enter. In most cases only one sperm 
cell enters the egg under normal conditions. This control is 
effected by the response of the egg to the first sperm. For no 
sooner has a sperm entered the egg than the latter develops a 
membrane which excludes all other sperms. 

702. Cleavage. After fertilization the eggs of higher plants 
and animals immediately begin segmenting (early cell division). 
Among the lower forms, on the contrary, a long "resting stage" 
often ensues after the zygote (conjugated gametes) or fertilized 
egg has formed a heavy membrane. 

703. In some instances the chromosomes of the two gametes 
can be separately followed through the first cell division. In 
which case it is seen that the chromosomes of the first segmen- 
tation spindle come in equal numbers from the two gametes 
and that therefrom the daughter cells receive an equal number 
of chromosomes from each parent. This is an important point 
which will be discussed more fully at another place. The events 
which follow the first cell division vary greatly with the organ- 
first division of the micronucleus; D, second division; E and F, three daughter 
nuclei disintegrate, the fourth divides into a migrating male element and a pas- 
sive female element; F and G, the male element migrates into the other cell and 
fuses with the opposite female element; H, the process completed; /, the cells 
separate. In maturation and fertilization (on the right) : A and B, the primary 
spermatocyte and the primary oocyte; C, secondary spermatocyte and secondary 
oocyte; Z>, mature egg and three polar bodies, and four sperms; E and F, sperm 
and egg unite and the nuclei fuse; G, the oosperm nucleus divides; H and /, the 
cell divides. A-D, maturation stages; E and F, fertilization; G-I, first 
cell division. In F-G the two cells are completely fused. Three of the sperms 
and the three polar bodies are not represented after stage E. 




FIG. 187. Six stages in the process of fertilization of the ovum of a mouse. 
After the first stage figured it is impossible to determine which of the two nuclei 
represents the male or female pronucleus. ek. Female pronucleus; rk\ and rk*, 
polar globules; spk, male pronucleus. (McMurrich from Sobotta.) 


ism, whether it is a unicellular form or whether it is a higher 
plant or animal. Only a few typical cases will be outlined. 

704. (i) When the cell has divided the daughter cells sepa- 
rate completely. On future divisions the same thing occurs 
and all the cells remain alike. This type includes the free 
protozoa and many unicellular plants. Sometimes a number 
of divisions occur before the cells separate, so that a group of 
eight, sixteen, etc., cells are produced. These then all become 
free at once and become independent organisms. 

705. (2) The cells divide, but are held together by a gelatin- 
ous matter which they secrete (Fig. 60) ; or by their cell walls 
(Figs. 182 and 212), or by a connecting bridge which develops 
into a branching stalk. These are colonies, and the arrange- 
ment of the individuals in the colony depends largely on whether 
the planes of division are always parallel, forming filaments 
(Figs. 182 and 212), or at right angles in two planes, form- 
ing plates, or at right angles in three planes, forming cubical 
masses. (Fig. 60.) 

706. (3) The cells divide structurally, but remain in func- 
tional unity. The resulting entity is not a colony, but still re- 
mains a single organism. Because of cell division the nuclear 
matter is distributed throughout the body of the organism, and 
the size of the body is not limited by the limited distance 
through which the nucleus acts on the cytoplasm, as would 
probably be the case if there were but a single central nucleus. 
The division of the body into cell units also permits differen- 
tiation to an unlimited degree, and with it division of labor 
and perfection of function. (See pp. 136, 338.) 

707. Under this head two types of development occur which 
we may characterize as evolving and involving. In the former 
the cell mass is solid and leads to the diffuse, plant type of 
organization, while in the latter the cells are arranged in layers 
enclosing cavities, and through it the compact animal type of 
organization is attained. (See pp. 328, 331 ff.) This apparent 



paradox disappears when we recognize that the " cavities" 
mentioned are not so much spaces as cleavage planes between 
organs, which permit the involution (growing inward) of other 

708. As examples of the first type we may take a fern and a 
dicotyledon. The fertilized egg cell of the fern divides into 
two cells, and these divide again, making four. These four 
cells continue to divide indefinitely, and the cells remain a 

FIG. 188. 

FIG. 189. 

FIG. 188. Development of the fern embryo. A, The egg cell divided into 
quadrants; B, a later stage, the four quadrants still evident and from them 
develop the four parts of the plant as indicated. /, Develops the foot; r, develops 
the root; s, develops the stem; /, develops the first leaf. 

FIG. 189. Development of the dicotyledon embryo, i and 2, Early stages 
showing all of the suspensor; 3 and 4, only the end of the suspensor is shown. 
In 2 the embryo is marked off at the upper end of the suspensor. In 3 the em- 
bryo is farther advanced. In 4 the root (T), stem (5) and cotyledons (C) are 

single solid mass. From each quadrant of the four-cell stage 
a definite part of the young plant develops; that is, from I 
develops the root, from II the stem, from III the first leaf, and 
from IV the foot, an organ by which the plantlet continues to 
draw nourishment from the mother prothallus. This foot is a 
type of structure which is very common among plants and ani- 
mals; that is, of structures which are functional only in the 


embryonic period of the organism, and later disappear or remain 
only as rudiments. 

709. In the dicotyledon embryo a similar embryonic structure 
is found. The suspensor is a row of cells which forms no 
part of the young plant. The terminal cell divides into two, 
four, eight, etc., cells, which at first form a spherical mass. 
This soon becomes heart-shaped and then the four parts of the 
dicotyledon embryo may be localized, viz. : I the caulicle, II 
the epicotyl, and III and IV the cotyledons. 

710. In many of the higher cryptogams the new cells are all 
divided off from one terminal or apical cell. In some cases a 
few divisions may occur in segments cut off from the apical 
cell, but only the apical cell is capable of continued division. 
In the dicotyledon, cell multiplication takes place only in limited 
regions of the bud and root-tip and in the cambium. These 
have already been described (p. 48). (Growth in Mono- 
cotyledons, see p. 343.) 

711. Types of Cleavage. The course of segmentation of 
animal eggs is greatly modified by the quantity of yolk present 
in the egg, and in this respect animal eggs vary greatly. Some 
eggs contain little yolk (Ccelenterates, Worms, Echinoderms, 
the lancelet, Mammals), others have moderate quantities (many 
Molluscs, some Fishes, Amphibia), while in still others the 
quantity of yolk may be many times in excess of the protoplasm 
(Arthropods, Cephalopods, many Fishes, Reptiles and Birds). 
The effect of the yolk is to retard or even completely inhibit 
division. The simplest type of development occurs, therefore, 
in eggs with little yolk, and as an example of this type we will 
take the egg of the lancelet. 

712. The Blastula. The first cleavage plane of the egg of 
the lancelet is vertical, the second is also vertical and at right 
angle to the first. The third is horizontal and cuts the four seg- 
ments slightly above the centres, so that the four lower cells are 
slightly larger than the four upper. This is due to the slightly 



(Figures 190 to 210 represent the early steps in the development of Amphioxous 
Branchiostoma). According to the wax models of Ziegler, Freiburg, i. B. 

FIG. 190. The mature egg with 
the polar body above. 

FIG. 191. Two-cell stage. 

FIG. 192. Four-cell stage. 

FIG. 193. Eight-cell stage. 

FIG. 194. Sixteen-cell stage. FIG. 195. Thirty-two cell stage, in section. 


33 1 

larger quantity of yolk at the lower pole of the egg. For the 
same reason the succeeding divisions take place slightly more 
rapidly at^the upper^pole of the egg, and hence the cells 

FIG. 196. Blastula. 

FIG. 197. Blastula, later stage. 

FIG. 198. Gastrulation. 

FIG. 199. Gastrulation, later stage. 

here are also somewhat smaller. The cells have a decided 
tendency to round up after division and so give the whole 
somewhat the appearance of a mulberry. Hence, this is called 



FIG. 200. 

FIG. 201. 

FIG. 202. 

FIG. 203. 

FIGS. 200 to 203 represent four steps in the development of the larva from the 
gastrula. The gastrula mouth narrows and is finally closed in by the ectoderm. 
The gastrula cavity elongates and becomes the primitive digestive cavity 


the mulberry or morula stage. In the centre of the mass a 
space is formed between the rounded inner surfaces of the cells, 
and this grows larger as division proceeds. The free sur- 
faces of the cells become flatter and they adhere more closely 
to their neighbors. By this way we arrive at a stage called the 
blastula, a hollow sphere, the wall of which is formed by a 
single layer of cells. The cavity of the sphere is the blastula 

713. The Gastrula. By the next series of changes the blas- 
tula is transformed into a gastrula. Cell division continues 
and the embryo increases in size, but we will fix our attention 
on another set of changes: First, the vegetative (yolk) pole 
of the blastula becomes flattened, then concave, as seen from 
the outside. This cavity deepens and thereby the blastula 
cavity grows smaller, until it is finally obliterated, when the 
inverted vegetative hemisphere comes in contact with the hem- 
isphere of the animal pole. The embryo now has the form of 
a cup, with double walls. This stage is the gastrula. The 
new cavity which has been formed is the gastrula cavity 
(archenteron), and its opening to the exterior is the gastrula 
mouth. The two layers which form the wall of the gastrula 
are the ectoderm and entoderm. After a time the embryo has 
elongated, and by unequal growth its axis has been shifted so 
that now the gastrula mouth, which has become very small, 
lies at the posterior dorsal extremity. 

714. The Medullary Plate. From now on several important 
developmental processes occur simultaneously, but we will 
trace them one by one. 

715. The dorsal surface of the elongated embryo becomes 
flattened and the cells of the ectoderm along the median line 
assume a columnar form, which results in a thickening of the 
ectoderm. The medullary plate thus formed curls up along 
its edges, forming a medullary groove. The edges of the groove 
approach above and fuse to form the medullary tube. This 



FIG. 204. FIG. 205. 

FIG. 204 is a cross section of the stage represented in Fig. 203. The medullary 
plate is shown above, between entoderm and ectoderm. 

FIG. 205. A later larval stage showing six mesodermic pockets, and the 
medullary plate completely covered by the ectoderm. 

FIG. 206. FIG. 207. 

FIG. 206 is a cross section through the anterior (left in the figure) end of the 
stage represented by Fig. 205. 

FIG. 207 is a similar section through the posterior end of the same stage. In 
both figures the medullary groove, the mesodermic somites and the first steps 
in the formation of the notochord are shown. 



FIG. 208. FIG. 209. 

FIG. 208 is a horizontal section through the stage represented by Fig. 205, 
and shows the six pairs of mesodermic somites. 

FIG. 209 is a section through a later stage (Fig. 210) and shows the medullary 
tube, the notochord, and the mesoderm pushing upward and downward between 
entoderm and ectoderm. 

FIG. 210. A later stage, showing most of the mesodermic somites completely 
cut off from the entoderm, the notochord, and the medullary tube. 


tube ultimately gives rise to the central nervous system of the 
animal. Before the tube is completely formed the edges of the 
ectoderm slip over it from each side toward the median line 
and fuse. This superficial part of the ectoderm gives origin 
to the epidermis with its modifications, and the sense organs of 
the skin. 

716. The Notochord. While the medullary plate is form- 
ing in the ectoderm the notochord and mesoderm are also 
taking origin from the entoderm. A longitudinal inverted 
groove is formed by the bulging upward of the entoderm along 
the mid-dorsal line. The edges of this groove unite to form a 
tube, which is then cut off from the entoderm. This tube 
develops into the notochord. 

717. The Mesoderm. At the same time two series of pock- 
ets are formed by the bulging outward of the entoderm in the 
dorsal lateral quarters, i. e., on either side of the notochord. 
These pockets also close and are cut off from the entoderm. 
They are called mesodermic somites, and are the first evidence 
of the segmentation of the body. They gradually extend down- 
ward and upward till they finally completely surround the 
medullary tube, the notochord and the remaining entoderm. 
The cavities of the mesodermic pockets develop into the body 

718. Other Types of Cleavage. So far as the segmentation 
stages are concerned the chief deviations from the lancelet 
type may be ascribed to the quantity and disposition of the 
yolk. In the lancelet egg, cleavage is total and equal. In the 
frog's egg there is a large amount of yolk accumulated largely 
at the vegetative pole. In consequence cleavage, though also 
total, is unequal and at the first horizontal division the cells of 
the vegetative pole are many times larger than those of the ani- 
mal pole. In the eggs of Cephalopods, many Fishes, Reptiles 
and Birds, the quantity of protoplasm is small compared with 
the yolk and forms a thin layer at the animal pole. When 


cleavage takes place the yolk does not divide and the cleavage 
only extends through the protoplasm, which is thus divided 
into a number of minute hummocks. From these, cells are 
later cut off by horizontal cleavage planes, and the same process 
extends outward and downward, continuously adding to the 
number of fully formed cells. This type of cleavage is called 
partial and discoidal, the latter referring to the disk-like form 
of the segmenting area. For most Arthropods cleavage follows 
still another course. Here the eggs are also laden with yolk, 
which is concentrated at the centre of the egg, while the proto- 
plasm lies more equally distributed over the entire surface. 
When the nucleus divides there is at first no division of the 
cytoplasm. But after a time the nuclei arrange themselves 
near the surface in a single layer. The cytoplasm then divides 
partially as in the preceding case, but over the whole surface 
of the egg. This type is distinguished as partial and superficial 

719. Origin of the Tissues. If allowance is made for the 
modification caused by the yolk, one may say that during the 
early stages of development all metazoa proceed along parallel 
lines. In all cases a blastula is formed, and this is followed by 
a gastrula stage. The formation of a distinct mesoderm, how- 
ever, does not occur in the Porifera and Ccelenterates. These 
animals, even in the adult, consist only of two distinct cell 
layers, the ectoderm and entoderm, though there is usually a 
supporting layer between. This may be simply a structureless 
membrane or, in some cases, a thick layer composed chiefly of 
' 5 gelatinous matrix. Some of the ectodermal and entodermal 
cells may send nervous or muscular fibre processes into this 
layer, and there may also be few or many cells of ectodermal or 
entodermal origin completely embedded in this layer. In the 
other phyla, where a true mesoderm is formed, there is consider- 
able variation in the method. It is almost always wholly ento- 
dermal in origin, but some times it begins as a solid outgrowth, 


which later splits to form the body cavity, and again it is gradu 
ally formed by cells which sink inward, one by one, from the 
entoderm and finally arrange themselves around the bod} 

720. In general, after the mesoderm and notochord have beer 
formed, the entoderm which remains forms the lining of th( 
digestive tract; that is, the mucous epithelium with all th( 
glandular organs connected with it, salivary, thymus, thyroid 
gastric, hepatic, pancreatic and intestinal glands. The mucous 
epithelium lining the tracheae, bronchi and lungs, is also oJ 
entodermal origin. 

721. The mesoderm gives rise to all other parts of the body 
the dermis, muscles, all connective tissues, including bones 
cartilage, teeth (in part), ligaments, tendons, fascia; heart 
blood vessels and blood; lymph, lymph glands, and spleen; the 
gonads and kidneys with their ducts; the pleura, pericardium, 
peritoneum and mesenteries. 

722. Indirect Development. The course pursued by the 
developing organism is not always the most direct. When 
the embryo gradually assumes the characters of the adult the 
development is said to be direct. Often, however, there is a 
sudden change in direction of development, so that the earlier 
steps seem to be directed toward a very different goal from that 
finally reached; so, for example, in most forms which are fixed, 
or very sluggish in the adult, there is an active free-swimming 
larva (Echinoderms, most Molluscs, Worms, Barnacles). This 
is called a dispersal larva because it seems to distribute the spe- 
cies partly by its own efforts, but more largely by the currents 
by which it is carried about. An interesting exception, which 
proves the rule, is that of the larvae of the fresh-water clam. 
If this were to follow the rule the larvae would be carried down 
stream by the currents much farther than the adult would be 
able to move upward during its entire existence. The result 
would be that the entire species would soon be carried to the 



sea. But this larvae attaches itself to the gills of fishes and may 
thus be carried up stream. 

723. Another type of larva is the trophic larva, like those of 
many insects. These are characterized by their voracious 
appetites. The caterpillar, for example, consumes much more 
than is necessary for its daily needs. The excess is stored up 
in the tissues as fat. When a certain stage of development is 
reached, feeding ceases. The caterpillar finds a suitable place 

FIG. 211. Development of the frog. An example of indirect development 
(metamorphosis). The gills have not disappeared in stage 6 but have been 
covered by a fold of the skin. (From Galloway after Brehm.) 

in which to rest. Here a complete change takes place. The 
skin is cast and there emerges a quiescent pupa, without func- 
tional appendages, without mouth or eyes. In this form the 
animal remains for a shorter or longer time, a few days to many 
months. Then another radical change occurs. The pupa skin 
is cast and the winged adult emerges. In many cases the adult 
feeds very little. Some have no functional mouth parts and 
would be unable to take food. They live for a short time, mate, 
and deposit eggs. Here the adult is the dispersal stage. Its 
organs of locomotion, the wings, are not for the ordinary pur- 


poses of securing food or escaping enemies as much as to 
facilitate mating and the carrying of the species to new and 
favorable localities. 

724. Differentiation of Germinal and Somatic Tissues. 
Reproduction in the protozoa and most unicellular plants (all 
unicellular organisms are some times grouped together under 
the name Protista) means merely a division of the body of the 
organism. The two halves resulting from division reorganize 
themselves, and after a period of growth, each has attained to 
the condition of the parent cell before division. This process 
continues indefinitely, and apparently there is no inherent 
reason why the substance of the Protist should not continue 
thus indefinitely; that is, the Protist organism does not naturally 
end in death. With the metazoan the case is entirely differ- 
ent. At a certain stage of development the body is divided 
into two classes of cells. A relatively small portion consists 
of cells destined to develop into gametes and, therefore, to con- 
tinue on in the succeeding generation. All the remaining cells 
of the body come to an end with the death of the individual. 
These two types of cells are distinguished as germ cells and 
somatic cells. This distinction rests on the principle of divi- 
sion of labor. Here, as everywhere else in the biological world, 
the chief end is the perpetuation of the species. This is 
secured more certainly if by unity of action but division of 
labor the life functions are performed in the most perfect 
manner. This seems to be the reason for the existence of the 
multicellular organism. 

727. Division of Labor and Differentiation. All the various 
types of aggregation of biological units are attempts to solve 
the same problem. The colonies of Protists, the hydroid 
colonies with polymorphism, the polymorphic societies of ants 
and bees, the various types of alternation of generation found 
among plants and animals are all devices to secure the most 
perfect functioning by dividing the functions. The meta- 


zoan individual is in one sense a colony of cells in which there 
is unity of action. But in order to secure the best results, the 
division of labor means differentiation, and differentiation 
carried very far means loss of power of recuperation and of cell 
division. Thus the functions of the somatic cells contribute 
to the perpetuation of the race by fostering the gametes and 
further nursing the embryo in many cases until it is well ad- 
vanced in development. 

726. Regeneration. That the loss of the power of recupera- 
tion goes hand in hand with differentiation, appears from a 
study of the phenomena of regeneration. An egg has the power 
of regeneration like that of a protozoan, so that from a fragment 
of an egg a complete embryo may develop. This is true even 
of Vertebrate eggs. Among the lower invertebrates such power 
of regeneration exists even in the adult. An anemone or star 
fish may be cut in two, and the parts will regenerate all the 
organs that were removed. This is not possible with a crab or 
crayfish. But even the Crustaceae will regenerate an append- 
age that was broken off. So as we pass to higher forms the 
power regularly decreases. In young Amphibia, appendages 
may still be regenerated, but in Birds and Mammals the power 
extends only to the healing of wounds, which is also a regenera- 
tion process. 

727. Mechanics of Growth. Differentiation begins at a very 
early stage. Even in the lancelet the entoderm is distin- 
guishable from the ectoderm before gastrulation takes place, 
and the cells of the medullary plate are distinguishable from 
the rest of the ectoderm before the medullary groove is formed. 
But it will not be necessary to describe the individual types of 
tissues here, since this was done in connection with the anatom- 
ical description of types. There are, however, some points 
about"* the mechanics of growth which should be noted. A 
naked ~ protoplasmic cell like Amoeba expands freely with 
growth_and often when there is a cell membrane it is elastic 



enough to expand, with the growth of its contents. But there 
are often rigid, supporting or protecting structures, which do 
not permit free expansion of the body. In such cases there are 

many interesting devices employed 
for securing expansion. The split- 
ting bark of the exogen type of stem 
has been described, as well as the 
growing root-tip and the bud. If 
the position of the apical cell and the 
angles of the planes in which the suc- 
cessive segments are cut off from it 
are carefully considered it will be 

OU U * seen that we have here also a device 

1 \*\ L-4 for securing freedom for growth. Let 
0Vl KM I I k us cons ider a ^ ew more cases among 
Ll ll I I plants. The diatoms are always 
unicellular, -and each cell is encased 
in a silicious capsule. The substance 
of which this capsule is composed is 
absolutely unyielding so far as the 
growth of the living contents is con- 
cerned. But the capsule is composed 
of two parts, which fit into each other 

FIG. 212. Mechanics of 
growth. A, A diatom; B, 
Microspora; C, (Edogonium. 
In A: a, the silicious " pill-box" 
shell of a diatom; b, a diatom 
dividing and forming two new 
half shells, back-to-back, within 
the old one. In B: a, b and c, 

three steps in the process of ri .-, r i ,. 

forming a new cross wall and llke the P arts of a Common gelatin 

capsule or a pill box, and can slide 
apart as the protoplasmic contents 
increase in volume. At the time of 

elongating the side walls of a 
dividing cell. In C: a, the cir- 
cular pad formed within the old 
wall preparatory to elongation; 
b, the old wall split under the 
pad and the pad stretching to 
form new wall; c, ridges left by 
a repetition of the process. 

division a new half capsule is formed 
inside each of the old half capsules. 
This means that each generation is 

confined in a slightly smaller compass than the preceding. 
Finally a limit is reached beyond which this decreasing size 
will not go. The shell is then cast off completely, a brief 
period of rapid growth as a naked cell ensues, and then a 


new and larger shell is produced as a starting point for a 
repetition of the process. 

728. Sometimes cells adhering in filaments are encased in 
thick tubular sheaths, which become too unyielding to keep 
pace with the growing contents by stretching. In the case of 
Tolyptothrix the tube splits at a certain point, the chain of 
cells breaks, and one end pushes past the other and out through 
the opening. This produces what is called false branching. 
Again in Oedogonium the unyielding tube in which the cells 
are encased is made to expand in a curious fashion. Inside 
the cell a circular cushion of new cell wall substance is formed 
against the inner surface of the old wall. This cushion com- 
pletely encircles the cylinder. Then the old wall breaks opposite 
the cushion, and this permits the latter to stretch and the cell 
to elongate, while at the same time maintaining the continuity 
of the cell wall. In Microspora the wall of the filament is made 
up of segments which are double-wedge-shaped in longitudinal 
section. These slip apart as growth proceeds and permit the 
insertion of new wedges by growth. 

729. Among the Monocotyledons two methods of common 
occurrence are of special interest. In this group of plants we 
find the sheathing leaf base a very common type. The sheath 
completely surrounds and protects the stem for some distance 
upward from the node. Within this sheath the stem remains 
for a long time meristematic, after the upper part of the same 
internode has completed its growth. This device permits the 
stem to continue growth longitudinally for a long time, but 
growth in thickness is practically completed when the upper 
end of the node appears above its sheath. This type is espe- 
cially characteristic of the long, slender, rapidly growing stems 
of grasses. A modification of this type occurs in the case of 
the perennial Monocotyledons like the palms. Here, the 
growth in height is very slow, and the short internodes are pro- 
tected for a long time by the basal portions of the leaf stalks, 



which often develop a very elaborate protective tissue. This | 
often persists long after the death of the leaf as a thick inter- | 
woven niat of tough fibres. Protected in this way the stem ! 
slowly increases in thickness for several years. When the pro- 
tective tissue finally rots away and exposes the stem the tissues j 

of the latter have reached their i 
final condition and the stem 
no longer expands in diameter. 
Thus the stem of this type 
soon reaches a limit in diameter 
while the growth in height 
may continue indefinitely, a 
condition which is decidedly 
inferior to that of the exogen 
stem, in which growth in thick- 
ness keeps pace with growth 
in height. 

730. For animals, the ques- 
tion of growth mechanics is 
fundamentally not as difficult 
as for plants, because the tis- 
sues are generally more yield- 
ing in character. At the same 
time the problem has appeared 
in much greater variety and 

FIG. 213. Section of a conch shell has been solved in more differ- 

entways. As soon as animals . 
covered themselves with a 

protecting shell they learned the trick of making that shell 
conical in form, so that by adding to the edge or mouth of the 
cone it grew wider as well as longer. This type of shell is 
found in many forms, from the protozoa to the cephalopods. 
But a flat cone offers less protection, while a long one is 
awkward to handle. This difficulty was also soon solved by 


coiling the cone into a spiral by more rapid growth on one 
side. This device is also very generally employed wherever 
the cone is in use (many Protozoa, some Worms, Gastropods, 
Cephalopods, especially extinct forms). 

731. One of the most distinctive characters of the entire 
phylum of Arthropods is the way the problem of growth me- 
chanics is solved. The Arthropod is entirely enclosed in a 
sheathing of chitin, a substance which is very elastic but has very 
little power of stretching. In fact, the animal cannot grow while 
encased in this armor. Consequently, the armor is removed peri- 
odically and then a period of rapid expansion ensues until the 
new shell has hardened again. Among the Crustaceae the casting 
of the shell (ecdysis) occurs frequently during the early periods 
of development (lobster 7-8 times in first year), later the moult- 
ing periods are less frequent (once per year, crab, lobster). 
During the soft-shell periods the animal remains concealed in 
some cranny, because it is then extremely helpless. Not only 
is it unprotected by a shell, but its " claws, " at other times so 
formidable, are now useless. Nevertheless, the Crustaceae as a 
class have been very successful, and we must conclude that the 
disadvantages of the period of ecdysis are more than compen- 
sated by the advantages of the chitinous armor. 

732. The more primitive Insects follow in general the Crus- 
taceae in regard to the management of this armor, but most 
orders of Insects have adopted a different and probably a better 
plan. Diptera, Coleoptera, Hymenoptera and Lepidoptera, the 
most numerous orders, develop by metamorphosis. Their 
larvae are soft-skinned and are in various ways enabled to dis- 
pense with the armor. During the pupal stage they are usually 
concealed in the earth or elsewhere, and after they emerge as 
completely armored insects, they no longer grow. Their growth 
is completed and no ecdysis is needed. Ecdysis occurs at the 
period of pupation, and again at the emergence of the imago, 
and at this time the insect is often concealed. 



733. The development of an internal skeleton by the Verte- 
brates called for a new solution of these growth problems. But 
in the Vertebrates numerous integumentary structures, each 
with its peculiar method of growth, are also found. We will 
only consider the epidermis here. This layer of the skin is con- 
stantly growing in all the terrestrial Vertebrates and its dead, 

FIG. 214. Ecdysis of the blue crab. The animal (lower part of the figure) 
has almost freed itself of the shell from which it escapes by backing out. 

outer layers are cast either at some intervals of time or as a con- 
tinuous process. In the Amphibia, this layer is cast at intervals 
in large or small patches. In Snakes it often comes away in a 
single piece. In Birds and Mammals the epidermis is constantly 
shedding in minute scales, but in these two classes there is 
usually a well-marked period of moulting or shedding of feathers 
and hair. When the epidermis is cast it is, of course, not the 



entire layer, nor even all the dead tissue. Only the hardened, 
more superficial part separates from the deeper, more flexible, 
layers. In some cases (Reptiles) a specially constructed layer 
of cells forms a cleavage plane. The process is comparable to 
the ecdysis in Arthropods, except that here we have to do with 
dead cells instead of formed substances. 

734. The method of growth of the bones varies greatly. In 
the smaller bones with simple form, the process is not specially 

FIG. 215. The carapace of the diamond back terrapin, Malaclemmys palus- 
tris. Note the concentric lines of growth in the horny plates. X 1/2. 

noteworthy, but with the "long" bones and those of com- 
plicated figure, the enlargement of the bone, and at the same 
time maintaining its form, is often a complicated process. For 
example, the skull of the adult is practically a single piece. 
This condition could not have been reached by the addition of 
layers of bone to the surface, since this would not provide for 
the growth of the brain unless, at the same time, the cavity of 


the skull were enlarged by the removal of material from the 
inner surface of the skull bones. The end is accomplished in 
another way. The skull is composed of many pieces, which are 
fitted together in a peculiar way. The seam, or suture, along 
which two bones join is not an even line or smooth joint; it is an 
extremely sinuous line which effects a dovetailing of the two 

FIG. 216. Skull of a human embryo at time of birth. The bones are still 
separated by seams of cartilage and membrane. The broad unossified space is 
called a fontanelle. In the figure the radiating lines on the parietal bone (large 
bone on the left) indicate the original centre of ossification and the direction of 

bones in a way to produce a very firm joint. The suture dis- 
appears at maturity by the complete fusion of the bones, but 
until the end of the growth period the suture is an open joint, in 
which material is being added to the bones of both sides. The 
skull, as a whole, therefore, expands by interstitial growth, while 



each individual bone increases its dimensions by the addition of 
material along the sutural surfaces. In Reptiles the sutures 
tend to remain open throughout life. 

735. In the ossification of the long bones like the femur, the 
bone is first deposited on the surface of the cartilage in its 

FIG. 217. Cross section of a vertebra of an embryo (pig) showing centres of 
ossification. The parts in black are cartilage. At three points the cartilage is 
being replaced by bone; in the centrum (A) and in the two sides of the neural 
arch (B). As the bony parts grow outward into the cartilage the cartilage 
between them also grows. Thus the vertebra increases in size with the growth 
of the body. Finally, however, all the cartilage is replaced by bone and the 
parts unite to form a single body of bone. 

middle region. The two ends remain cartilaginous for some time 
and grow by the growth of the cartilage. In the middle region 
or shaft the growth of bone continues by the addition of new 
layers to the outside, and the new layers extend gradually 




further toward the two ends. After a time new centres of 
ossification occur at each end. These form bony discs, which 
are later separated from the shaft only by a thin seam of carti- 
lage. In this seam there are, however, three zones of growth ; a 
middle zone, in which cartilage is rapidly forming, and on either 
side of this is a zone in which the cartilage is being eroded and 
replaced by bone. It is thus that the shaft increases in length, 
while the epiphysis is also increasing in thickness (compare 
with the growth in the cambium ring) . When growth is com- 
plete the epiphysis unites with the shaft. 

736. This method by which a complicated skeletal figure 
expands by interstitial growth through the growth of parts is 
also found among invertebrates. An excellent example is in 
the test of the sea urchin. 

737. In cases where the proper form cannot be secured 
through growth by the addition of material to earlier stages, 
it often happens that parts of the earlier structure are actually 
removed, so that growth consists in a process of tearing down 
and building larger. This takes place, for instance, in many 
gastropod shells which form a thick rounded fold at the mouth 
of the shell at the close of the seasonal growth period. At the 
beginning of the next growing season this fold is removed by 
absorption before the edge of the shell is extended. (Ex. : The 
queen conch.) This also very often occurs in the development 
of the Vertebrate skeleton. The central cavity of the shaft of 
an adult femur, for example, is much larger than it was when 
the first layer of bone was laid down on the cartilage. Hence, 
the bone must have been removed at a later period. So also 
the lower jaw of the embryo, with its complex curvature, can- 
not be included within the outline of an adult jaw. There 

FIG. 218. Three successive steps in the growth of the queen conch (Cassia). 
The thickened lip of the shell (+) in A, is shown in B (+) partly absorbed and 
overgrown by the new growth. A new Up (0) is formed after a period of growth 
and this is again partly absorbed and overgrown (o, in C). In C, nine or ten 
successive stages of growth may be counted by the remnants of the lips. X 1/2. 



must have been a process of absorption at work as development 

738. Progressive and Regressive Development. Wherever 
development is direct the organization of the body is a con- 
tinuous progressive process toward the final perfect adult. But 
when there is a change in the course of development, as in all 

FIG. 219. Sexual dimorphism in a beetle, Cladognathus. The difference 
appears both in size and in the peculiar development of the mandibles of the 
male. Male on the left. In many beetles the male is larger than the female. 

cases of metamorphosis, or where there is a radical change in 
the life habits of the animal, there is also a break in the con- 
tinuity of development, and to a greater or lesser degree a re- 
versal of development. This rs the case, for example, when a 
free-swimming larva becomes fixed in the adult, or when a holo- 


zoic larva becomes paraistic in the adult. Such changes in- 
volve a loss of function, or at least a change of function of some 
organs, and hence a change in the organs themselves. This is 
called regressive development. When the tadpole develops 
legs and lungs and leaves the water, some of its organs have 
become useless. We need mention only the gills and the broad 
fish-like tail. These organs, being now no longer needed, un- 
dergo regressive changes, they are gradually resorbed, dwindle 

FIG. 220. Male (left) and female (right) of a fire-fly, Lampyris. The male has 
well-developed wings but the female is wingless. X2. 

and completely disappear. It must not be inferred, however, 
that if any tadpole were kept in the water that these changes 
would not occur. Indeed, these organs have become useless 
before the frog leaves the water. The position might be assumed 
that the change of habit occurs because of the change in 
organization. (See p. 339.) 

739. Sexual Dimorphism. In some species the adult in- 
dividuals all strictly conform to one type. This is exceptional, 
however, and applies only to hermaphrodyte forms like the 
earthworms and some snails. The vastly more common con- 
dition is a sexual dimorphism; that is, two types of individuals 


are regularly developed, male and female. The difference be- 
tween the two is often indistinguishable except in the gonads, 
and it may require dissection to determine the sex. In other 
cases the gonads are visible through the transparent wall of the 
body, and a difference in color of those organs often distinguishes 
the sexes (jelly fish, some worms, etc.). More frequently there 
are secondary sexual differences, such as accessory sexual 
organs, egg-laying apparatus, or copulating organs or structures 
which are more remotely or not at all connected with the function 
of reproduction. The female is very generally larger than the 
male, a fact which is probably to be connected with her greater 
trophic functions. This is notably the case among insects. 
In a few remarkable instances the male is minute, compared 
with the female, and may even be attached to her as a parasite. 
(Ex.: Barnacles, Sacculina, Oedogonium.) Among Mammals 
the males often fight with each other for the possession of the 
females, and this has resulted in a greater development in size 
and strength of the males. The male of the fur seal is four 
times larger than the female. Among Birds the difference be- 
tween the sexes is most conspicuous with regard to coloration 
and song, in which the males usually far excel the females. 
(See p. 423.) Among butterflies there are often remarkable 
differences in coloration between the sexes, and in a number of 
Insects the male is winged while the female is without wings 
(glowworm and Hibernia moth). Among plants, sexual 
dimorphism is usually evident only in the accessory reproductive 
organs (flowers). 

740. Polymorphism. There is also a manifolding of form 
types which has no direct relation to sex. It is best developed 
in lower forms, especially those which are colonial. Among the 
Hydrozoa there may be as many as four or five types of indi- 
viduals. These may be classed as the trophic or feeding polyps, 
the budding polyps, the protective polyps, and the sexual me- 
dusae. In the Siphonophores there are, in addition, the swim- 



-.. b 

s b 

r.z. J 

FIG. 221. Diagram of a Siphonophore colony, b, Float; *.&, swimming bell- 
m, mouth; w.z., trophic polyp; p.z., protective zooid; rz\ rz\ n* reproductive 
zooids; t, tentacles. (From Galloway, after Lang.) 



ming bells. The vibraculae and avicularia of the Bryozoa, are 
also modified zooids. In these cases, which are characterized 
as stock polymorphism, the differentiation of individuals occurs 
in connection with division of labor, and in this special type 
could only occur in a colony. An analogous kind of polymor- 
phism occurs in the social Hymenoptera, the bees and ants, and 
the termites among the Corrodentia. In these societies there 



FIG. 223. Hydractinia, a polymorphic hydroid. C, Ccenosarc covering the 
substratum; n, trophic polyps; r, reproductive polyps bearing buds containing 
ova; t, tentacles. (From Galloway after Hincks.) 

are males, females and workers. The latter are undeveloped 
females. In some ants and termites there is also a fourth class, 
the soldier, individuals with exceptionally large heads and 
formidable jaw. The interdependence of the individuals of 
these societies is almost as great as that of polyps in a 
Hydroid colony. 

741. Alternation of Generations. When polymorphic types 
alternate with each other in successive generations we have the 



common phenomenon of alternation of generations. This 
occurs in the Hydroid colony when the free-swimming sexual 
medusa originates by budding from a colony, and itself gives 
rise to a new colony by a sexual method. This type of polymor- 
phism is well exemplified by many of the trematodes (see p. 368), 
and is particularly widespread among plants. Indeed, all plants 
above the thallophytes undergo a regular alternation of genera- 
tions. In the higher forms it is rather obscure and not easily 

FIG. 223. Polymorphism in Termes lucifugus. A, Adult worker; B, soldier. 
Both A and B are undeveloped males or females. C, Perfect insect (male or 
female); D, same after shedding the wings; E, young complementary queen; F, 
older complementary queen. Enlarged. (From Folsom after Grassi and 

described. In Mosses it is most conspicuous. The leafy 
moss plant develops from a spore and is itself sexual and de- 
velops eggs and spermatozoids. From these are developed the 
spore capsule with its stalk. These remain connected with the 
sexual plant, but are themselves the asexual generation by 
which the spores are produced. 

742. There are still other types of polymorphism of less com- 
mon occurrence. Seasonal dimorphism occurs, for example, 
among some butterflies. In this the broods produced at diff- 
erent seasons are often very differently colored, so that there 
are summer andjall types or wet'andjiry season types. 



743. The polymorphism found in many flowers, as a device 
for securing cross fertilization, has already been described. A 

FIG. 224. Passage ways of the "white-ants" in a post. The termites avoid 
the light, ordinarily, and hence construct tunnels of mud to cover their runways. 
These tunnels are often very extensive and much labor is involved in their con- 
struction. This is performed by the numerous small workers. Xi/2. 

very peculiar type of polymorphism occurs among spiders and 
butterflies. A number of species of spider are known to pro- 



duce two kinds of males, and among butterflies there occurs a 
duplication of types of females. In one case at least there are 
said to be five kinds of females. (See p. 430.) 

744. Life Habits Depending on Food. The character of 
the food and the method by which it is obtained exercises a 

FIG. 225. Seasonal dimorphism in a butterfly (Prioneris) from India. A, Wet 
season form; B, dry season form. Xs/4. 

profound influence upon the organization of the body. The 
typical plant absorbs C0 2 and various mineral salts and through 
photosynthesis builds up its tissues. Such plants are said to 
be holophytic. The typical animal ingests organic matter and 



prepares it for absorption and assimilation by digestion. Such 
an animal is said to be holozoic. 

745. Some times two organisms of different kinds are found 
living together by mutual consent, apparently, and partake of 
the same food. The sea anemone, on the shell of the hermit 

FIG. 226. Seasonal dimorphism in a European butterfly, Araschnia levana. 
Both are females: A, the winter form; B, the summer form. X 2. 

crab, is often quoted as an example of this kind. The anemone 
secures fragments of the crab's food, and the crab secures some 
measure of protection by the presence of the anemone. Such a 
relationship is called commensalism. More frequently the 
needs of two organisms are to some extent complementary, and 
one household may serve both to mutual advantage. This is 



FIG. 227. An example of a complex interrelationship of organisms. Three 
large brown ants (Camponotus?) are guarding a small colony of aphids from 
which they obtain honey-dew; the abdomens, of these three ants being greatly 
distended with what they obtained in this way. The day after the above sketch 
was made the large ants had been driven away by a large band of small black 
ants, which then took possession of the aphid colony. The aphids in this case 
are feeding on a fungus (Peridermium?) which, in turn is parasitic in the bark 
of the trunk of a pine tree. Ants are known to care for the eggs of aphids during 
the winter, and carry the young to appropriate food plants, and then guard the 
aphid colony from the attacks of other predatory insects. For this service the 
aphids pay a tax in honey-dew. The honey-dew is a clear, sweet fluid secreted 
in drops from the anus (not from the tubules on the dorsal surface of the abdo- 
men). The ants stimulate the aphids by stroking them with their antennae; 

to this thp. anViirl<; rp^nnnH Vv voirlintr n rlrrmlpf r>f flip 


symbiosis, and, as examples, we may refer to the green hydra, 
the green fresh- water sponge and some Protozoa in which cells of 
a unicellular alga have found the conditions of life favorable 
within the protoplasm of the animal host. The CO 2 eliminated 
by the animal tissues is food for the plant, while the O eliminated 
by the plant cells is equally useful to the animal. A similar 
relation exists between algae and fungi of many kinds in the 
group of organisms called Lichens. Here the algae are wound 
about by the mycelium of the fungus so that they seem to form 
a single organism. It has been found, however, that they may 
be separated and grown independently of each other, and in 
their structure they show their identity or near relationship 
with algae and fungi which are found in nature unconnected. 
The mutual advantage here is probably like that in the case of 
hydra, and in addition the alga is protected from the dry air 
by the dense tissue of the fungus, and the fungus possibly 
secures soluble food from the alga. 

746. It is difficult to judge of the degree of helpfulness or 
harm which one organism exercises over another in such a 
common household. A long list of examples like the following 
might be enumerated: The little oyster crab which is found 
at home within the shell of the oyster. A similar crab is found 
in the tube with certain marine annelids (Chaetopterus). 
Certain ants capture the cocoons of other ants and rear the 
young as slaves. Among ant colonies are found a variety 
of other insects living in more or less harmony, though not 
always by the consent of the ants. Among the tentacles of 
certain jelly fish (Cyanaea) a small fish is usually found. 
Among higher animals a companionship between birds and 
mammals is often observed. 

747. Parasitism. More commonly the relationships of this 
sort are decidedly disadvantageous to the one party. This 
is then called parasitism, which in several respects is one of the 


most important biological phenomena, and merits extended 

748. The common mildews, lilac or grape mildew, which 
are seen in late summer as a whitish "fur" on the surface of 
leaves, is due to a mildew spore which, blown by the wind, 
falls upon the leaf and germinates. It puts out a slender tube 
which grows through a stoma into the mesophyll. Here it 

FIG. 228. Peridermium, a rust fungus parasitic on pine trees. The white 
ridges are composed of masses of spores. X2/3. 

develops by absorbing its nourishment from the mesophyll 
cells, until finally it puts numerous branches out through the 
stomata, and on each of these are borne numerous spores. 
The orange-colored or black specks which appear later on the 
surface of the leaf are spore cases in which a second kind of 
spore is produced from the same mycelium. 

749. The rusts which occur on our cereal grasses, wheat, 


oats, etc., so much, have a complicated life history. One of 
the most complicated is that of the common wheat rust. In 
the spring the winter spores (teleutospores) 
germinate, produce a short mycelium on which 
four small spores of another kind (sporidia) are 
borne. These germinate on the surface of the 
barberry leaf, enter the mesophyll by the 
stomata and develop a mycelium. What is 
called a cluster cup is then formed just under 
the lower epidermis. This is filled with spores 
(aecidiospores), and when the epidermis finally 
breaks, the spores are set free. These then 
germinate on the wheat leaf and in its tissues 
a fourth kind of spore appears in such masses 
as finally to burst the epidermis and produce 
long, narrow orange-colored pustules filled with 
summer spores (uredospores). These may 
germinate in the same way and produce new 
generations of uredospofes. Later in the season 
still another kind of spore appears among the 
summer spores, or in clusters by itself. These 
have thick, dark-colored walls, and make black 
patches on the leaf. These are the winter 
spores, teleutospores, which germinate in the 
next spring and start a new cycle. 

750. The common cedar apple is the teleuto- 
spore-bearirig stage of a rust which has its 
cluster cup on the leaves of the haw. 

751. These parasites are fungi. They are 
typical plant parasites and often greatly damage 
the host, as, e. g., the wheat rust. Less im- 
portant, economically, are the phenogamous 

parasites. The Indian Pipes are common flowering plants 
growing on the roots of other plants. The parasite has no 

FIG. 229. A 
fungus, Cordy- 
ceps ravenelii, 
parasitic in the 
grub of a beetle, 
L a c h n o s terna. 
Two long stro- 
mata of the 
fungus are seen 
growing from the 
body of the grub. 
(From Folsom 
after Riley.) 



chlorophyll and the leaves are scale-like. Only the flowers 
are like those of normal holoplytic plants. The dodder (love 
vine, golden thread) is a curious example. When the seed 
germinates on the ground a slender, leafless stem grows out. 
It does not root in the ground, but lies flat on the surface. In 
that way it continues to grow at one end, and if necessary, at 

FIG. 230. Dodder, or golden thread (Cuscuta). The weed host is completely 
overspun by the parasite. The flowers and seed pods of the dodder are seen in 
great numbers. Xi/2. 

the same time, absorbs its substance at the other end, so that 
it grows along without having yet received any nourishment 
except what was contained in the seed. When by this creep- 
ing along the ground it comes in contact with certain green 
plants, it attaches itself to them. It sends little root-like 


structures (haustoria) into the stem of the host and from its 
tissues absorbs nourishment. Then it continues to grow, 
climbing upon the host and winding about from stem to branch, 
and from one plant to another, until a veritable tangle of golden 
threads is spun about the hosts. The leaves of the dodder are 
minute scales. It has no chlorophyll and its mode of nutrition 
is wholly parasitic. Small white flowers are finally produced and 
seeds as in normal holophytes. Our American "mistletoe" 
is only partially parasitic. 

752. Among animals we likewise find parasitism more com- 
mon among lower forms. Among Vertebrates only the round- 
mouth eel, the lowest of fish-like forms, deserves the name 
parasite. The sponges and Echinodermes contain no parasites ; 
Ccelenterates and Molluscs very few. The unsegmented worms 
are the largest contributors to the list, and Insects follow closely. 
The Cestodes and Trematodes are the most common internal 
parasites, and to the order Hemiptera belong most of the 
external parasites. 

753. The Trematodes are in some respects comparable with 
the rusts, especially with regard to the complicated life history, 
multiplicity of methods of reproduction and tendency to alter- 
nate hosts. As an example often described we may take the 
liver fluke. 

754. The adult fluke lives in the liver of the sheep and matures 
many thousands of eggs, which pass down the bile ducts into the 
intestine, and thus to the exterior. From these eggs hatches 
a free-swimming larva, provided the eggs, washed by the 
rain, or by some other means, reach a pond or stream. The 
larva has a pair of eye spots, but is otherwise very simply 
organized. Its further development depends upon its coming 
in contact with a certain species of fresh- water snail. This pro- 
vided, it attaches itself and bores its way into the interior 
of the snail, where it continues to develop as a parasite. It 
loses eyes and cilia (sense organs and locomotor organs) and is 


little more than a sack (sporosac), in which, by a process of 

internal budding, a number of new individuals, rediae, are pro- 

duced. These are slightly more 

highly organized, but continue 

the process of internal budding 

for several generations. Then 

the rediae, by a similar process, 

develop a new type of individ- 

uals called cercariae. These are 

again a little more complex. 

They have two suckers, a forked 

intestine and a tail. The cer- 

cariae leave the snail and be- 

come encysted on the grass. 

If now they happen to be in- 

gested by a sheep with the grass 

they are set free from the cyst 

in the stomach of the iiost. 

The parasite may now be called 

a young fluke, for if it succeeds 

in finding the opening of the 

bile duct it works its way up 

into the liver and then develops 

directly into the fluke. The 

mature fluke is well organized 

as far as digestive tract and 

reproduction Systems are COn- 

cerned. The reproductive sys- 


FIG. 231. The liver fluke, Fasaola 
hepatica, showing the arrangement of 

tern especially is very highly cirrus sac ; 0, mouth; Ov, oviduct, or 

, . uterus; S, ventral sucker; Sg, shell 

developed. gland; T, testis; U, intestine; 7, 

755. AcommonCestodeisthe ^^ } duct - (From Tyson .after 
tapeworm. The one common 

in the dog may be taken as a type. The eggs originate in the 
intestine of the dog and reach the earth with the faeces. Either 

3 68 


through the drinking water or blown in the dust upon the 
food, these eggs find their way into the stomach of the rabbit. 

FIG. 232. Diagram of the life history of'the liver'fluke (Fasciola). A, Egg; 
B, embryo; C, ciliated larva;*/), sporocyst; , sporocyst, later stage; F, mature 
redia containing young rediae and cercariae; G, cercaria; H, same encysted. /, 
young fluke; b, brain; b.p, birth pore; c, cercaria; c.m., cell masses which develop 
into embryos; e, eye-spots; ex., excretory tubules; g, intestine; m, mouth; ph, 
pharynx; r, redia; s, suckers; sc, sporocyst; +, stages at which non-sexual re- 
production occurs; *, stage of sexual reproduction. (From Galloway after 
Thomas, Leuckart, and others.) 

Here the larvae are set free and bore their way into the tissues 
of the stomach, then get into the blood vessels and ultimately 



become fixed in the muscle or other tissues of the body. The 
larva develops into a bladder-like, cysticercus, in which are 
formed one or more embryonic scoleces. In this condition it 
remains until the flesh in which it is embedded is eaten by a dog. 
Then the scoleces are set free. They attach themselves to the 
wall of the intestine by means of the suckers and hooks, and 
then develop the tapeworm strobila. In this case the develop- 

FIG. 233. Diagram of the tapeworm, Taenia. A, Cysticercus or bladder- 
worm stage. B, later stage of same. C, Strobila. The last proglottis shows the 
uterus which is filled with embryos; D, one of the embryos in the egg shell; b, 
bladder; ex, excretory canals; g, genital pore; h, scolex with hooks and suckers 
(s); , uterus; z, zone of strobilation. Some of the proglottides are numbered; 
many are omitted. (From Galloway.) 

ment is simpler than that of the fluke, and asexual multiplication 
may be confined to the strobila, as when the cysticercus de- 
velops only one scolex. There is, however, always an alter- 
nation of hosts. There are many kinds of tapeworms, each 
with its specific two hosts. Thus, one tapeworm of the dog 
finds its other host in the dog flea. Others alternate between 
cat and mouse, goose and crayfish, man and fish, man and swine, 
man and dog, etc. The tapeworm is a dangerous parasite, not 


so much in the adult stage as in the cysticercus. This, embedded 
in the brain or other organs of the body, sometimes reaches 
enormous size and destroys the surrounding tissues of the host. 
756. As internal parasites the threadworms are very common. 
The famous "horsehair snake" (Gordius) is a parasite in the 

FIG. 234. Sexually mature proglottis of Taenia. ov, Ovaries; rs, receptaculum 
seminis; sg, shell gland; t, testis; v, vagina; vd, vas deferens; yg, yolk gland. 
Other letters as in preceding figure. (From Galloway.) 

cricket during the later developmental stages. At maturity it 
is free, living in the water. 

757. Trichinella spiralis is the parasite which often infests 
the flesh of swine and is frequently transmitted to man by the 
eating of uncooked pork. The adult worm is 3-4 mm. long, 
and bores into the wall of the intestine, where the young are 
produced in large numbers. The young are only .1 mm. long, 


and are carried by the lymph and blood to the muscle, where 
they remain and grow to a length of i. mm., and become en- 
closed in a capsule. Unless flesh containing such encapsuled 
trichina is eaten by another mammal, the development of the 
worm proceeds no further. When such flesh is eaten the cap- 
sule is dissolved and the half-grown worm is set free. Thus it 
gets into the intestine and reaches maturity in the walls of the 
intestine. This parasite may infest any of the flesh-eating 
domestic mammals. 

758. There are a large number of intestinal parasites belong- 
ing to the round worms which infest the intestine of all the do- 
mestic animals and man. Some of these (Ascaris) reach the 
size of an earthworm and are very prolific. The eggs find their 
way into the digestive tract of a host with the water and food, 
and their development takes place wholly within the intestinal 
cavity. These seldom produce an extreme pathological condi- 
tion in the host. 

759. Many smaller threadworms are parasitic in plants. 

760. The parasitic Arthropods are chiefly found in two or 
three orders. Among the Entomostraca are the fish lice, chiefly 
external parasites, and the specially notable case of Sacculina. 
The adult Sacculina is found attached to the ventral surface of 
a crab. The portion of it, which is visible externally, is little 
more than a large sack containing an elaborately developed 
reproductive system. The sack is attached to the host by a 
process of its body, which penetrates the tissues of the host, and 
then branches and penetrates in all directions, like the root 
system of a plant. By this organ it absorbs nourishment from 
the host. It has no digestive system. The young of this strange 
organism are free-swimming nauplii with eyes and appendages 
of the typical nauplius. But when the larvae attach them- 
selves to a host, the appendages and eyes undergo degenera- 
tion until there remains only the organism as described. 

761. Among Insects, the parasites are found chiefly among 

37 2 


the Diptera and Hemiptera. The larvae of the Diptera are often 
parasites. The botfly, Hypoderma, develops under the skin of 
cattle. The botfly of the horse (Gastrophilus) deposits its eggs 
about the shoulders and head of the horse. The horse gnaws 
them off or they fall into his food, and thus get into the stomach, 
where the larvae remain attached to the wall of the stomach. 
762. The botfly larvae of Cephalomyia ovis in the frontal 
sinuses of sheep produce blind-staggers. The larvae of the 

FIG. 235. Ichneumon fly, Thalessa lunator, depositing eggs in the burrow of 
the wood-boring Tremex upon whose larvae the larvae of the Thalessa feed. 

ichneumon fly, a Hymenopter, are -parasitic in the cater- 
pillars of various butterflies. The adults of a large number of 
flies are temporarily external parasites, as are also many mos- 
quitoes. The flea is also closely related to the flies, and its 
wingless condition is probably the result of degeneration 
through parasitism. 

763. The Hemiptera, or bugs, are, as a group, parasitic. 
They are often evil smelling because of the secretion of a peculiar 
gland. The bed bug and squash-bugs exemplify this point well. 
The plant lice, scale insects, the water striders, water boatmen 



and electric light bugs (the last three are aquatic bugs), the 
cicadas (harvest fly and seventeen-year-locust), and chinch bugs, 
are all familiar parasites, largely on plant hosts. The Phyllox- 
era is parasitic on the grape, and merits detailed description. 
In the spring the first generation of young hatch from eggs which 
were deposited the preceding fall under the bark of the vine. 
This generation is wingless and reproduces parthenogenet- 
ically. Successive generations of similar individuals follow. 

FIG. 236. A tomato worm covered with the cocoons of its parasite, Apanteles, 
which is also a Hymenopter. (From Folsom.) 

These cause the galls on the leaves and the nodules on the roots, 
for they also attack the roots underground. In late summer 
another type appears. These are winged and serve to scatter 
the species. They lay two kinds of parthenogetic eggs on the 
underside of the leaves. From the larger eggs there develop 
females, and from the smaller ones males. These are both 
destitute of digestive tract. The females, after fertilization, 
deposit a single egg under the bark of the vine. These eggs re- 
main over winter and hatch the first generation in the spring. 



764. The galls so often seen on oak leaves and twigs, and also 
on many ather plants, are abnormal developments of the plant 
tissue due to a stimulation produced by insect parasites. The 
female Cynips, a wasp-like insect, deposits her eggs in the tissues 
of the plant, and during the development of the young the tissues 

FIG. 237. Dog flea, Ctenocephalus canis. A, Larva; B, adult, 
after Kunckel d'Herculais.) 

(From Folsom 

B A 

FIG. 238. Oak galls (.4) made by the gall wasp, Holcapsis globulus (B). 
A y natural size; B, magnified X 2. (From Folsom.) 

are irritated in such a way as to cause the abnormal develop- 
ment of the surrounding tissues. The gall forms a shelter for 
the young brood and the juices of the plant provide food. 

765. Protozoa As Parasites. Many species of amoeba 
(Entamceba) are found, as parasites, in the digestive tract and 


in other organs of the body. They have been found in \many 
mammals, birds, frogs, and insects. Some of these scarcely de- 
serve the name parasite, since their presence in the digestive 
tract seems to cause the host no inconvenience. To this class 
belongs Entamceba coli, which is found in the human intestine 
in a large percent, of normal individuals. Entamceba histoly- 
tica, however, penetrates the wall of the intestine and causes 
the disintegration of the tissues, or ulceration. This is the 
cause of tropical dysentery, a serious and often fatal disease, 
which is quite common among the people of tropical countries. 
766. Among the Flagellates the Trypanosomes are the most 
important group of parasites. They find their hosts among all 

FIG. 239. A Trypanosome. /, Flagellum; m, undulating membrane; n, nucleus. 
(From Marshall after Doflein.) 

the classes of Vertebrates, as well as some invertebrates, but 
the Mammals are most seriously affected. The parasite is 
usually found in the blood and causes intermittent fever, swelling 
of the spleen and lymph glands, anaemia, eruptions of the skin 
and disorders of the nervous system. Trypanosoma gambiense 
is the cause of the terrible "sleeping sickness" of South Africa. 
It is apparently the toxic effect of the parasite on the nervous 
system that produces the later symptoms of the disease, a 
lethargic condition which slowly leads to a continuous sleeping 
and finally ends in death. In large parts of South Africa the 


cattle, horses, and in fact, all the domestic mammals, as well 
as wild mammals, are affected by a disease known as Tsetse 
fever. It is fatal to such a degree that " large areas are closed 
to colonization" where the disease is endemic. Trypanosoma 
Brucei is the cause of the fever, but Tsetse is the name of a fly. 
The natives have long known that the fever only occurs in 
districts in which the Tsetse fly is found, and there is now no 
doubt that this fly, in stinging affected cattle becomes itself 
infected and then carries the germs to uninfected cattle. There 
are several species of Tsetse fly (Glossinia), and of these, prob- 
ably more than one is responsible for the spread of Tsetse fever. 
The sleeping sickness is also carried by Tsetse flies. 

767. The domestic animals of South America, southern 
Europe and northern Africa, and the countries bordering on 
the Indian Ocean, are also affected by different types of Trypano- 
some diseases. In these cases other flies and mosquitos are 
the principal agencies of infection, but lice and fleas may per- 
form the same office. The Trypanosomes of fishes are carried 
by leeches. 

768. Of more direct interest to us is the parasite of malarial 
fever. There are at least three varieties of this, producing the 
" tropical, " the " tertian, " and the "quartan" fevers, respec- 
tively. At a certain stage there are found numerous minute 
bodies floating in the blood plasma of the host. These are 
the "spore" stage of a Sporozoan, Plasmodium. They are 
vastly smaller than a red blood corpuscle and are capable of 
amoeboid motion. They attach themselves to a red corpuscle 
and work their way into it. Here they grow at the expense of 
the blood corpuscle, and at the end of 48 hours, in the case of 
the tertian parasite, they divide into a number of "spores." 
Hereupon the corpuscle goes to pieces and the spores are again 
floating in the plasma. These spores repeat the cycle just de- 
scribed and thus a new generation of "spores" is produced on 
each alternate day. This process may continue indefinitely, 



FIG. 240. Life history of malarial parasite, Plasmodium. i, Sporozoite 
introduced into human blood by bite of mosquito; 2, same a little later; 3 and 
4, same growing in a red blood-corpuscle; 5, same dividing; 6, blood-corpuscle 
disintegrated and setting free the spores; 7, 8 and 9, a spore developing into a 
female gamete; 70, 8a, ga and gb, a spore developing into a number of male 
gametes; 10, union of male and female gametes (fertilization); n, motile zygote; 
12, zygote embedded in the wall of the stomach of the mosquito; 13 to 1 6, stages 
in the development of sporozoites in the sporocyst; 17, sporozoites in the salivary 
gland of the mosquito. Stages from i to 8 in the human blood. Stages 8 to 17 
in the mosquito. (From Folsom after Grassi and Leuckart.) 


but at intervals another type of development occurs side by 
side with the spores. In this case the enlarged parasitic cell in 
the blood corpuscle does not divide into a number of spores, but 
becomes much elongated and cresent-shaped. Now, however, 
for further development a change of host is necessary and this 
host must be one of a few species of mosquitos (Anopheles). 
In the stomach of the mosquito the crescents just described be- 
come differentiated into two classes. In the one class the 
crescents become rounded and motionless, while in the other 
division occurs and a number of very long and slender motile 
bodies are formed. These are female and male gametes re- 
spectively, and a fusion takes place between them as in fertili- 
zation. The zygote now becomes motile. It works its way 
into the wall of the digestive tract where it remains for about 
eight days while undergoing further development. This is 
called the sporocyst stage. During this period it grows to an 
enormous size, it first divides into a number of cells and these 
then each develop a vast number of sporozoites, very long and 
very slender spindle-shaped motile spores. By the bursting 
of the sporocyst the spores escape into the body cavity and thus 
gain access to the salivary glands. For some reason they work 
their way into the salivary glands and their ducts, and hence, 
when the mosquito next punctures the skin of an uninfected 
person some of the sporozoites are carried with the saliva into 
the wound, and the victim is thus inoculated with malarial 
virus. The sporozoites are merely another type of spore. 
They attack the red blood corpuscles in the same way as in the 
stage with which we began. 

769. We see that the malarial parasite completes its life 
history only by transferring from man to mosquito. Both 
hosts are necessary. This is probably also true of the try- 
panosomes, as is indicated by the more recent investigations. 

770. The fever days of malaria are the days in which the 
new generation of spores are set free by the disintegration of the 


blood corpuscles. This occurs every other day in tertian fever, 
and on every third day in quartan fever. A double inoculation 
may result in a more complicated succession of fever days. 

771. The Apes, Bats and Birds are also subject to Plas- 
modium parasites. Texas cattle fever is caused by a elated 
Sporozoan (Babesia), which is transmitted by the cattle tick. 

772. Bacteria as Parasites. Most infectious diseases are 
caused by bacteria. This has been definitely established for 
many diseases, but the difficulties in the way of determining a 
causal relationship between such minute organisms and the 
diseases with which they are supposed to be associated are 
often very great, and in a number of cases the organism has 
not yet been identified, though the disease is almost certainly 
known to be bacterial. The part of the body infested by the 
parasites varies with the species; sometimes it is the mucous sur- 
faces of the digestive tract or the respiratory passages, some- 
times the tissues of certain organs, sometimes the blood vessels 
and lymph spaces of certain organs or even of the entire body. 
The mode of infection also varies with the disease. Some- 
times the germs find their way to the host with the food, water 
or air of respiration, but they may also enter the body through 
the skin. The latter is not likely to occur except when the skin 
is broken. 

773. The effect of the parasite on the host is sometimes 
limited to a disorganization of the tissues of a limited region. 
The result of this may not be serious, but if the destruction 
of the tissues goes far in a vital organ the function of the organ 
may be seriously impaired and result in the death of the host. 
In other cases no anatomical change can be observed in the 
tissues, and yet the function of some organs may be disturbed 
and consequently the life of the host threatened. 

774. Bacteria vary like other classes of parasites with regard 
to the range of hosts in which they may be found. But this 
question has been studied more particularly from the point of 


view of the host and has led to several very important general 
conclusions concerning the degree of susceptibility of species or 
of individuals to given bacterial diseases. From what is said 
concerning the physiological processes of bacteria in the Ap- 
pendix to Part I, it may be suspected that the effect produced 
on the host by the bacterial parasite is due to substances 
secreted by the bacteria. As a matter of fact, it is found that 
if an extract from the bacterial cultures containing no living 
cells is introduced into the body of the host, the symptoms 
peculiar to the corresponding disease are produced. The bac- 
terial products are similar to poisons in their effects, and are 
called toxins. 

775. Immunity. When an individual or a species is not 
susceptible to the attack of an infectious disease, it is said to be 
immune. The immunity may be a native character of the 
animal, it may be acquired during the life time of the individ- 
ual through natural causes, or it may be induced artificially. 
These types are, therefore, designated natural immunity and 
acquired immunity, respectively, and of the latter there are 
two types, active and passive. 

776. Natural immunity is due primarily to three kinds of 
defense, which the organism employs to defend itself against 
bacteria which have succeeded in entering the body, (i) Any 
foreign particles introduced into the tissues and causing irri- 
tation are attacked by the white blood corpuscles. These 
cells are capable of independent locomotion through amoeboid 
movements. They escape from the blood vessels by pene- 
trating the walls and move about in the lymph spaces in the 
tissues. They collect about foreign matter and engulf and 
digest particles as would an amoeba. This process of phagocy- 
tosis is regarded as of great importance in keeping the body 
free of bacterial invasions. (2) The blood of a naturally 
immune animal contains a substance, alexin, which causes the 
death of the bacteria. This substance is probably formed by 


the cells of the various tissues. (3) It is generally true that 
individuals are not equally susceptible to tht same poisons and 
natural immunity rests, in part, on this fact. The immune 
individual is not affected by the bacterial products which act as 
toxins in other individuals. This is accounted for by the pres- 
ence in the blood of the immune individuals of a substance, 
which neutralizes the toxins, and is, therefore, called antitoxin. 

777. The animal organism is not passive to the attacks of 
bacteria. If the attack is not too sudden and violent the 
tissues respond by producing antitoxin, which neutralizes the 
effect of the toxin, and a lysin (alexin), which causes the dis- 
solution of the bacteria. Another substance is also formed 
which causes the bacteria to adhere in clumps. This is called 
agglutinin. These substances (anti-bodies) produced by the 
tissues in response to the bacterial stimulus, may continue to be 
formed long after the exciting cause has disappeared and the 
body is therefore immune to a second attack. This is known 
as acquired immunity. 

778. Bacteria are extremely variable. This is especially 
evident in the degree of virulence of different strains of what is 
apparently the same species. Immunity acquired from the 
attack of a mild form is also generally efficient against the more 
virulent types. This principle is employed to produce immunity 
by intentionally infecting or inoculating an individual with a 
mild type and thereby securing protection against more danger- 
ous forms. Immunity secured in this way is active acquired 

779. Passive immunity is secured by injecting into the 
animal a blood serum obtained from an immune animal. 
This serum-therapy is effective at the time, but the anti- 
bodies soon disappear from the blood and, since the tis- 
sues have not been stimulated to the formation of anti- 
bodies, the immunity is lost. 

780. The various types of immunity may be illustrated by 


a few familiar forms. Man is naturally immune to fowl cholera, 
though sometimes attacked by cattle fever, anthrax. The fowl 
is immune to rabies, to which both man and the dog are subject. 
The dog is immune to. anthrax. By an attack of measles or 
smallpox man acquires immunity against subsequent attacks. 
By vaccinating man with the virus of cowpox, a mild disease 
is produced, which renders the individual actively immune 
against smallpox. Passive immunity against diphtheria is se- 
cured by the injection of an antitoxin serum taken from an 
actively immune horse. 


781. Species. The word species is one of the most important 
and most frequently employed of all biological terms, and yet 
it is impossible of exact definition. A species is a kind of a 
plant or animal, using the word kind in its most common sense. 
Thus the sweet-gum (Liquidambar Styraciflua), the persimmon 
(Diospyros Virginiana), and the tulip tree (Liriodendron tulipi- 
fera), are clearly defined species as are also, among animals, the 
turkey vulture, or buzzard (Cathartes aura) and the robin 
(Turdus migratorius) . Any given example may immediately 
be recognized as sweet-gum or not-sweet-gum, as robin or not- 
robin. In other cases, however, difficulties may arise. Thus, 
among the oaks we have the willow oak, the blackjack, the white 
oak and the Spanish oak, all of which are distinct species and 
readily distinguishable. But it frequently occurs that the 
flowers of one species are pollinated by those of another and the 
resulting offspring is called a hybrid. It resembles both parent 
species to some extent, but belongs to neither. A much more 
important difficulty arises from individual variation, for the 
individuals of a species are never exactly alike. Even the in- 
dividuals sprung from the same parents may vary greatly 
among themselves. Allowance must, therefore, be made for 


individual variation within the species. Sometimes certain 
types of variation occur so constantly that the species may be 
subdivided into varieties. This is especially true where a 
species is widely distributed and thus lives under different 
conditions in the various districts it inhabits. Where there 
is variation corresponding to geographical distribution it is 
called a geographical variety. A good example of this is 
Lycaena pseudargiolus, the common small blue butterfly,, which 
ranges from New England to Arizona. The New England, 

FIG. 241. Two flower heads of Gaillardia. The head on the right is the nor- 
mal type, with ray flowers ligulate. The head on the left is a variation (sport) 
which frequently occurs. The ray flowers in this are tubular and often quite 
regular. X2/3. 

Middle, Southern and Southwestern States varieties of this 
butterfly differ so much that they might well be classed as four 
distinct species, were it not for the intergrading forms found 
in the transition regions. Nor is this an isolated example. 
Extended study of a species almost invariably widens the range 
of its recognized variability, and sharply defined species are the 
exception rather than the rule. 

782. Opposed to the tendency to vary is a tendency for the 
species to maintain its character. This is evident in the re- 
semblance of the offspring to the parents. Although the in- 


(Third filial generation). 

FIG. 242. Mendelian inheritance in the four-o'clock. If red (a) and white 
(b) forms are crossed the offspring are all pink (c). These interbred yield 1/4 
red (d), 2/4 pink (e and/), and 1/4 white (g). The lower part of the figure 
shows the condition of the zygote and the gametes in the ancestral and first, 
second, and third filial generations. (From Davenport after Haecker.) 


dividuals of the same brood differ among themselves and from 
their parents they still resemble their parents, and hence each 
other, more than they do distant relatives or unrelated members 
of the same species. That is to say, the peculiarities of the 
parents tend to reappear in the offspring. This, in some cases, 
has been found to be controlled by very simple laws, known as 
Mendel's Laws of Heredity. If the two parents differ with 
regard to a certain character the offspring of the first generation 
will inherit equally from both parents. In the second genera- 
tion one-fourth will have the original paternal character only, 
one-fourth will have the original maternal character only, and 
the remaining two-fourths will still be mixed like the first gen- 
eration. In the third generation the mixed two-fourths of the 
preceding generation will be divided into a fourth pure paternal, 
a fourth pure maternal, and two-fourths mixed. In the fourth, 
fifth, etc., generations this process will continue. In this it 
must be recognized that a character may be present, though not 
evident. Such a character is said to be recessive while the op- 
posed character, which is evident, is said to be dominant. To 
illustrate we may take a simple case. If the red and white 
varieties of Mirabilis Jalapa (four o'clock) are crossed, the off- 
spring in the first generation are all pink. The second genera- 
tion (secured by close fertilization of the pink generation), 
however, consists of three kinds, viz.: One-fourth white, two- 
fourths pink, and one-fourth red. The white and red forms 
are said to be pure because they continue to produce only white 
and red, respectively, in succeeding generations if close fertil- 
ized. The other two-fourths of pink flowers in the next 
generation again break up into white, pink, and red forms in 
the proportion of 1:2:1, as before, and thus the pink or mixed 
forms continue in each generation to separate into the three 

783. When the two parental characters are not equally 
potent, i. e., when one is dominant, the other recessive, the 


results are as follows: We will take as an example the pure 
white and common gray mice. The result of this cross is 
gray mice like the gray parent, not a lighter gray, as one might 
expect, following the case of Mirabilis Jalapa. In the second 
generation there are a fourth white, which are pure, and three- 
fourths are gray. The gray are in reality of two kinds, though 
this becomes evident only in the course of succeeding genera- 
tions, when it develops that one-third of the three-fourths 
continue to breed only gray, while the other two-thirds yield 
one-fourth white in the succeeding generation and are thus 
seen to have been mixed. The second generation of offspring 
may, therefore, be described as one-fourth white, two-fourths 
mixed with gray dominant, and one-fourth pure gray. The 
dominant grays and pure grays can only be distinguished by 
the character of their offspring. 

784. Not every character is controlled in this simple way, 
for it is readily conceivable that a given character may be due 
to the combined operation of several factors, each of which may 
be separately heretible. 

785. Physical Basis of Heredity. The phenomena of he- 
redity correspond in a remarkable way with those of matura- 
tion, which makes plausible the theory that the chromosomes 
are the bearers of the hereditary traits of the organism, and 
that during maturation the readjustment of chromatin deter- 
mines the ancestral characters which are to be handed on to the 
next generation. In the process of fertilization the germ cell 
is provided with equal parts of maternal and paternal chroma- 
tin, and this condition is maintained during the subsequent 
stages of development in all the cells of the body. Let the 
condition of the chromatin with regard to any hybrid character 
be represented by (P.M.). Then when the first maturation 
division occurs in the primary oocyte of the hybrid (F.) the 

/T * \ i j- -j j. { (P.M.) = ist polar body 

(P.M.) elements divide into ? ;* /; 

( (P.M.) = secondary oocyte. 


But when the reduction division occurs the paternal and 
maternal elements are separated and the result is ........... 

......... either .................... or 

J (P.M.) j = ist p. 'b.= f (P.M.) 1 
2dp.b.= { (P.) (M.) j =mature eggs= { (P.)(M.) j = 2 d p. b. 
If the first polar body divides the result is 

^ \ /i/\ r in. which either one of 

(P.) (M.) j 

the four cells may represent the ripe egg and the other 
three the three polar bodies. The ova are, therefore, either 
(P)aternal or (M)aternal in regard to the hybrid character, and 
the two kinds are probably equally numerous. 

786. In the maturation of the sperm the process is similar. 
The primary spermatocyte (P'.M'.) divides into two secondary 

(P' M' ) 

, ' /' which then divide into 

/p/\ /;*f/ \ f ur spermatids. Two of these are (P')ater- 

nal and two are (M')aternal, so that the number of (P'.) and 
(M 7 .) sperm is also equal, as in the case of the egg. 

787. By close fertilizing, the gametes of the FI hybrid 
generation may combine as follows: 

(P.P'.), (P.M'.), (P'.M.), (M'.M.). This results in one-fourth 
pure paternal, two-fourths hybrid and one-fourth pure maternal 
individuals in the F 2 generation. 

788. Number of Species. In spite of the fact that it is 
often extremely difficult and even impossible to definitely cir- 
cumscribe a species, yet for convenience of reference animals 
and plants must be divided into convenient groups, named and 
classified. And much time has been spent in hunting out and 
describing species. About 520,000 animal species have thus 
been listed and over 200,000 plants. Besides these there are 
many species which have become extinct and are now only 


represented by fossil remains. Of these there are about 60,000 
known. In addition to these there are many species living 
which have not been observed by the recording biologist, and 
hence do not appear in the count, and there have probably 
been many, many more which became extinct without leaving 
any trace or the remains of which have not yet been found. It 
is, therefore, not probable that any one familiar with the facts 
would regard a million a high estimate for the number of species 
which have been or are now living on the earth. 

789. Origin of Species. The question naturally arises, 
Whence came they all? It is a question which has always 
occupied thinking men, and concerning which there has been 
much difference of opinion. To-day biologists generally, if 
not all, are of the opinion that species are plastic, as it were, and 
continually undergoing modification, so that they are not to-day 
what they were or what they will be, and further that two sec- 
tions of a species may become modified in different directions 
and thus come to differ even to the extent of specific distinction. 
In this way there would arise two species where there had been 
but one. This is known as the theory of the origin of species 
by descent with modification. In connection with this theory 
there are two questions which should be clearly distinguished. 
The first one- is as to fact, the other as to method: (i) What 
is the evidence that species originate by descent with modi- 
fication? (2) If a fact, how does it come about? In the fol- 
lowing pages we will consider the evidence upon which the 
theory of descent rests, and in that connection take up a number 
of important biological phenomena which have not yet been 

790. The Taxonomic Series. Long ago, students of natural 
history were struck by the fact that animals could be arranged 
in a series in the order of their various degrees of organization; 
with the simplest at one end, the most complex at the other, and 
the interval between more or less completely filled by forms of 


intermediate grade. This series is fairly well represented by 
almost any scheme of classification adopted by systematists 
to-day. Great similarity in form and structure naturally 
suggests a blood relationship and a common origin. But the 
taxonomic series represents a continuous chain of such relations, 
and hence leads to the conclusion that the entire animal king- 
dom had a common origin, and that by a process of evolution 
the higher forms have developed from lower forms as the com- 
plex adult develops from the simple egg. 

791. The taxonomic series is, however, not adequately 
represented by a line and it is now more frequently compared 
to a tree. At its base this genealogical tree divides into two 
trunks, one representing the vegetable kingdom, the other the 
animal. The base from which they both spring represents the 
many unicellular forms which have both vegetable and animal 
characteristics. Then up along the animal trunk come the less 
differentiated forms, like some Ccelenterates, Annelids, Peri- 
patus, Branchiostoma, etc. The branches represent the highly 
differentiated forms and spring from the main stem at various 
points; the Sponges below, higher, the Echinoderms, Arthro- 
pods, Molluscs, etc., while several large branches at the top 
represent bony Fishes, Reptiles, Birds and Mammals. 

792. With the advancement of the study of anatomy much 
information has been obtained which throws light on this ques- 
tion. In the vertebral column of Vertebrates, for example, 
we have a structure which indicates clearly a relationship 
between the five classes of Vertebrates, but when we examine 
the skull and the appendages we seem at first to' have only 
hopeless diversity. But here also a wonderful uniformity 
appears on more careful investigation. With regard to the 
skeletal portions, the wing of a bird, the fore limb of a bat, the 
flipper of a whale, the fore legs of the horse and dog and the arm 
of man, are all constructed of the same elements and each one 
of these appendages may be homologized bone for bone with 


the others. It must be kept in mind that bones may fuse, may 
dwindle by degeneration to the point of disappearance, and even 
new bones may develop, which have no counterpart in other 
animals. But such modifications do not detract from the value 
of homologies. In the same way the skulls of these five classes 
are also found to belong to the same type, and one can homolo- 
gize the bones of a frog's skull with those of a dog. The resem- 
blances between Birds and Reptiles are especially numerous in 
parts of the skeleton, but a detailed knowledge of comparative 
osteology is necessary to a full appreciation of such points. 

793. The plant kingdom, as a whole, does not form as perfect 
or continuous a series as the animal kingdom, but all the 
important breaks in the series fall below the Archegoniates. 
The simple structure of the Algae and Fungi offers compara- 
tively few features for comparison, and hence makes it difficult 
to discern relationship. On the other hand, the series from 
the liverworts to the highest flowering plants is more perfect 
and more extensive than anything to be found among animals. 
When we trace the gradual development of the sporophyte and 
the corresponding reduction of the gametophyte, through the 
various classes of Bryophyta, Pteridophyta and Spermatophyta 
we are able to form an almost ideally perfect series. When one 
studies this series, not only as a whole but in its details, the con- 
viction that it represents a " blood relationship" is irresistible. 

794. The anatomist often discovers organs which are clearly 
without function. It is impossible to account for such organs 
except on the ground that they are rudiments of organs which 
were at one time functional. That such is the case is usually 
evident by comparison with other species in which homologous 
organs are to be found. When an organ becomes useless for 
any reason it still persists in a more or less imperfect condition 
as a vestige of its former state. Some examples of vestigeal 
organs are of particular interest in the present connection.!" 

795. The whale is a mammal, since it suckles its young, as 


well as for many other reasons. But it has only two appendages, 
the flippers, which represent the fore limbs. No evidence of 
hind limbs is to be found externally, but on dissection a rudi- 
mentary pelvic girdle may be found, and in the Greenland 
whale there are also rudimentary femurs and tibias. The 

FIG. 243. The "Congo Snake," Amphiuma means. This animal is not a 
reptile but an amphibian. It is found in the southern United States. Note 
the rudimentary appendages. Xi/3- 

Ophidia are classed as an order of Reptilia, but they have no 
appendages. Yet in the case of the python there is a rudi- 
mentary pelvis and also rudimentary appendages which appear 
at the surface as horny points. Most lizards have two pairs of 


functional appendages, but in several species the appendages are 
more or less rudimentary. In one the fore limbs are entirely 
wanting, while the hind ones are greatly reduced. In still an- 
other form there are no limbs at all, but both girdles are present. 
In all these cases we have animals which are classed with quad- 
rupedal forms, though they do not have four legs. The sig- 
nificance of such rudimentary structures cannot be overlooked. 

FIG. 244. The female luna-moth, Tropaea lima, seen from beneath. The 
abdomen becomes so heavy before the eggs are deposited that the moth is 
unable to fly. XL 

796. The young baleen whale has rudimentary teeth which 
never develop to a point where they can be of service to the 
animal. In the middle of the upper surface of the skull of 
many lizards there is a small opening. Over this place the skin 
is transparent, and beneath there is an eye which is connected 
with the fore brain by a long stalk. In the Cyclostomes there 


is a similar eye in a functional condition, but in all other Verte- 
brates it is wanting. In its place there is an organ whose func- 
tion is not known but which is homologous with the pineal eye 
of Cyclostomes and lizards, and is probably a functionless 
rudiment. The rudimentary paired eyes of cave fishes belong 
in the same category. 

797. As an example, from among invertebrates the wings 
of insects may be mentioned. Most insects have wings, but 
the order Aptera contains no winged forms. In this group 
there are no rudimentary wings or other evidence that the 
insects ever possessed wings. The order Hemiptera contains 

FIG. 245. Hibernia marginaria, a species of moth in which the female has 
wings much reduced and useless. A, Male; B, female. Xi 1/2. 

many wingless forms, but many others are winged and many 
have rudimentary wings. In this case the wingless forms are 
regarded as representing a degenerate condition. In other 
orders there is also evidence of degeneration of wings. The 
male gipsy moth flies well. The female is also provided with 
well-developed wings, but she never uses them. Among the 
species of the geometrid moth genus, Hibernia, the females are, 
in some cases, wholly without wings, while in others various 
stages in the reduction of wings may be found. A similar 
condition exists in the beetle family, Lampyridae, the common 


"fire flies." Here the males are always good fliers, and in some 
species the females are also, but in other species the females 
have rudimentary wings or the wings are entirely wanting. 

798. Among plants the rudimentary leaves and other organs 
to which frequent reference has been made are further examples 
of the principle under discussion. 

799. In the first series of examples cited in this section we 
saw how organs adapted for such diverse purposes as swimming, 
walking, flying and writing may be constructed on a common 

FIG. 246. Hibernia defoliaria. The female is wingless. See preceding 
figure. A, Male; B, female. Xi 1/2. 

plan. No plausible explanation for this remarkable fact has 
ever been offered except that of a common origin. If these 
animals had a common pentadactyl ancestor the present 
diversity as to the condition of their appendages is the result 
of modification in different directions as a result of different 
conditions of environment. 

800. The vestigeal organs also can only be accounted for 
on the supposition that the ancestral forms possessed the organs 
in a functional condition, and that changed conditions, involv- 
ing a disuse of the organs, resulted in their degeneration. This 


means then also that the rudimentary organ is an indication 
of kinship with forms in which it exists in a functional condition. 

801. If this principle is granted the kinship of the entire 
organic world can be more or less completely established. 

802. The Phylogenetic Series. A large part of the rocks 
of the surface of the earth were formed by the deposits of mud, 
sand and organic remains under water. Thus originated the 
shales, sandstones, and limestones. While the rocks were 
being deposited the bodies of animals and plants were frequently 
buried and the resistant parts preserved as fossils. Of course, 
the rocks which were formed first are now beneath those which 
were deposited later. By the fossils and their relative posi- 
tion in the rocks we have been able to learn something of the 
character of the former inhabitants of the earth and of the 
order in which they appeared. The geologist has been able to 
reconstruct in considerable detail the more recent periods of 
the earth's history, but the earlier chapters are difficult to 

803. The Cambrian period is the earliest in which we find 
any evidence of life. It was a period covering a vast extent of 
time, and at its close most of the invertebrate phyla, if not all, 
were already represented. The highest Molluscs, the Cephalo- 
pods, were present, and also the aquatic Arthropods, the Crus- 
tacea. Insects and spiders appeared in the next period, the 
Silurian, and also the first Vertebrates, Fishes. In the Devonian 
period Fishes were extremely abundant. The Amphibians and 
Reptiles appeared in the Carboniferous period. Mammals 
did not appear until the Jurassic period, and Birds came still 
later, in the Triassic. So far as this evidence goes it shows that 
the lower forms appeared first. With regard to Vertebrates, 
more particularly, the order in which the classes appeared also 
agrees with what one would expect according to the theory of 

804. The fact that Birds appeared after Mammals must not 



be misinterpreted. Of two forms, the one which appeared later 
is not necessarily descended from the other, since both may 
have arisen independently from still earlier forms. And the 
one which appeared latest is not necessarily the highest. Birds 
are as highly specialized as Mammals. But the mammalian 
type of specialization may be described as a more successful 
one, and Mammals are, therefore, usually placed above Birds. 

805. The geological record is very fragmentary and only 
occasionally do we get a connected story. The history of the 
snail, Paludina, has been worked out in detail, and we have an 

FIG. 247. Fossil remains of ArchaEOpteryx lithographica. 
(From Galloway, after Claus.) 

account of the changes through which the genus passed during 
a considerable interval of time. The shell, which was at first 
very simple, with smooth, rounded contours, became step by 
step angular and ribbed. So that the later species have little 
resemblance to the earlier forms. 

806. The first bird of which we have any record is the 
Archaeopteryx. This bird had a long tail consisting of a series 
of vertebrae, fringed on either side with feathers. The wings 
were provided with three free digits armed with claws. The 
head was very large and had heavy jaws, both of which were 




provided with a complete series of teeth. Some of the birds 
of the Cretaceous period were much more like modern birds, 
but still had teeth on certain parts of the jaws. The Archaeop- 
teryx was almost as much reptile as bird, and even the later 
types presented many decided reptilian characters. 

807. The most complete series of mammalian fossils are those 
which show the genesis of the modern horse. The earliest 
mammals were pentadactyl (Phenacodus) . The earliest horse- 
like form had four toes on the fore foot and three on the hind 
foot. From this we see the number of toes gradually reduced, 
until in the modern horse there is only one functional digit and 
small rudiments of the second and fourth. The first digit (I) 
was the first to disappear and then the fifth (V). These were 
then followed by the second and fourth. 

808. Geological evidence concerning the history of the 
development of the vegetable kingdom is much like that for 
animals. The Cryptogams existed in great profusion long be- 
fore the seed-bearing plants appeared, and the Gymnosperms 
preceded the Angiosperms. 

809. The Ontogenetic Series. The differences between indi- 
viduals are the greatest when the individuals are mature. The 
young, the late embryos and the earlier embryonic stages are 
successively more and more alike, and differences finally vanish 
in the egg. Hence all metazoa start from the same level. In 
fact, the differences which exist up to the end of gastrulation 
are of secondary importance and have no relation to the sys- 
tematic rank of the developing organism. All metazoa are, 
therefore, in a real sense alike up to the end of gastrulation. 

8 10. Suppose an observer entirely unacquainted with the 
characteristics of eggs were given a series of eggs representing, 
we will say, Ccelenterates, Annelids, the lancelet, a fish, a bird 
and a rabbit. In the eggs themselves the observer would find 
no means of determining the class to which they belonged. If 
now these eggs each passed through its appropriate develop- 


mental processes the first identification would be possible after 
gastrulation, because at this point the ccelenterate development 
ceases. The other embryos continue their development by the 
formation of the third layer, the mesoderm. The body becomes 
elongated and the mesoderm assumes the form of a series of 
mesodermic somites by which the body is divided into a homono- 
mous series of metameric segments. Here the annelid larva 
has attained the form of the adult worm, and hence the end of 
its development. The other larvae develop gill slits, and at this 
stage the development of the lancelet is virtually completed. 
The further addition of large cerebral vesicles and paired eyes 
would indicate vertebrate embryos, the fish, the bird and the 
mammal. But only the bird and the mammal would develop 
lungs and pentadactyl appendages. In the bird the aortic 
arch would lie on the right side of the body, while in the mammal 
it would be on the left. 

811. In this comparison of the development of animals 
details are, of course, omitted. The purpose of the comparison 
is to show that the difference in the result of development of the 
lower and higher forms is due not to a difference in direction of 
development so much as to its extent. All forms pursue the 
same course, but the higher forms continue their development 
farther. Stating the same thing in another way, we may say 
that the higher forms, in their development, pass through 
stages which are the permanent adult condition of lower forms. 
Why? If it is granted that the baleen whale is descended 
from a terrestrial quadruped with the dentition characteristic 
of Mammalia, then the rudimentary teeth of the young whale 
are a relic of the former condition, and in its development the 
whale passes beyond the tooth-bearing stage to a stage of rudi- 
mentary teeth. This argument applies to all rudimentary or 
vestigeal organs. The same line of reasoning may be applied 
to another type of development in which an organ, instead of 
remaining rudimentary, passes beyond the normal type. 


For example, the wing of the bat is supported chiefly by the 
enormously developed phalanges of the II, III, IV, and V 
digits. In the embryo the bat hand is at first a normal pen- 
tadactyl hand, and the great elongation of the four fingers 
does not take place until very late. The bat being also a 
Mammal follows the mammalian type of development, and 
after it has reached the grade of a Mammal it continues its 
development into the bat stage by transforming the mammalian 
appendage into the more specialized bat appendage. 

812. If the above is a true conception of the origin of vesti- 
geal and highly specialized organs then we are also in a position 
to understand the parallelism of development described above. 
It has been stated as a "Fundamental Law of Biogenesis" that 
"the development of the individual recapitulates the history of 
the race," which means that in its development each organism 
passes through stages which represent the adult condition of 
ancestral forms. 

813. The fish has four or five pairs of gill slits and between 
the slits are the gill arches which bear the gills. Farther back 
in the mid-ventral line lies the heart, from which a vessel runs 
forward and divides into as many pairs of vessels as there are 
pairs of gill arches. These vessels go one to each gill arch, and 
above as many vessels pass from the arches to the mid-dorsal 
line, where they unite into a single vessel, the dorsal aorta. 
By these vessels the blood passes from the heart over the gills 
for respiration. In the embryo of Amphibia, Reptiles, Birds 
and Mammals, we also find these gill slits and the same arrange- 
ment of blood vessels. In Amphibia this fish-like condition 
persists in a functional manner until the time of metamorphosis 
of the tadpole. But in the other three classes respiration by 
gills never occurs and the gill slits are functionless rudiments. 
For the arrangement of the several pairs of vessels which pass 
over these functionless gill arches there is also no explanation 
to be offered except that they have been inherited from fish-like 



ancestors. The blood passes through these vessels for a time, 
but soon an entire rearrangement takes place, and in Birds and 
Mammals the connection between the heart and dorsal aorta 

FIG. 249. Diagram of the human embryo to show the arrangement of the 
blood-vessels. E, Eye; 0, ear; Mn, lower jaw; H, heart. From the heart a 
large vessel leads forward and then divides into five pairs of vessels (only the 
five of one side are represented). These vessels pass over the gill arches to the 
dorsal side and there unite to form the dorsal aorta. In the adult, however, 
only the fourth branch of the left side remains to connect the heart with the 
dorsal aorta. (From McMurrich after His.) 

is maintained only by a single vessel. In Birds this vessel 
lies on the right side, in Mammals on the left. The other vessels 
either become connected with other parts of the body or else 



fail to develop at all. In adult Amphibia and Reptiles a pair of 
these vessels persists to form the aortic arches. 

814. The conclusion seems evident. The gill arches and 
their blood vessels are a fish character, and their presence in the 
terrestrial vertebrates can only mean that as vestigeal organs 
they hark back to fish-like ancestors. But this is only one of 
many anatomical puzzles which can be explained in this way. 

815. The remarkable parallelism which appears between 
the Taxonomic series, the Phylogenetic series and the On- 
togenetic series assuredly warrants the formation of a provis- 
ional hypothesis of the origin of species by descent. The 
value of any hypothesis is gauged by the extent to which it 
explains phenomena and by the help it gives in the discovery 
of new facts. We shall proceed to apply this test, but let 
us first enquire what causes might be supposed to bring 
about a change in species. 

8 1 6. The Struggle for Existence. Taking the whole world 
into account and year after year, there is on the average no 
great change in the number of individual organisms. Locally 
and for brief periods there frequently occurs an increase or a 
decrease in the number of a given species. But extended 
changes of this kind are comparatively rare even for a single 
species. This indicates that each pair of adult individuals 
at the time of death have provided a progeny of the same num- 
ber to fill the gap. But the rate at which even the slowest 
breeding animals reproduce is much in excess of this, and for 
many the rate is many thousand-fold greater. Many plants 
produce several thousand to several million seeds in a season 
and, in the case of perennials, this is done for many years. 
For many animals a single brood of eggs ranges from many 
thousands to many millions. The conger eel may produce five 
or six million eggs, while the female Ascaris is credited with a 
brood of sixty-four million eggs. Yet in these cases only one 
or two eggs can ultimately have developed into an individual 


of reproductive age. The others must in some way be destroyed. 
Many seeds and eggs are devoured by animals and many are 
not brought into an environment favorable for development. 
But one need not observe very closely to discover that the 
number of young is always much greater than the number of 
adults. The Nemesis of destruction follows the young through- 
out the period of development, and indeed throughout life, 
but the ratio of mortality is greatest during the earlier 

817. Now what is it that determines which one of a thousand 
young should survive? Is it merely a matter of chance or is 
there a difference between individuals which gives certain ones 
an advantage? Let us consider an imaginary concrete example. 
Suppose a litter of young rabbits in a nest at the edge of a forest. 
These young are more or less like their parents (heredity), 
but are not all exactly alike (individual variation). One soon 
shows itself to be puny, is ill-nourished, and perhaps falls a 
prey to disease or, being less active, is the first to fall a prey to 
the weasel or other predacious animal. Its more active mates 
escape the first assault, but one is particularly light in color 
and is readily seen at night in the open field, where both he 
and the owl are seeking their food. Perhaps one is too dark and 
his color fails to blend with the dead grass where he attempts 
to hide. One may be rather stupid. He fails to sense the 
enemy until it is too late. But among the rest there is one 
just the right color, that of his successful parents. He is alert, 
strong of limb and a nimble dodger. He runs fast, dodges 
quickly and has the instinct to hide at the right time and place. 
He is the one most likely to survive to the season when he estab- 
lishes a family of his own. 

8 1 8. This is a purely imaginary case, but no unreasonable 
supposition is made. That rabbits vary in regard to such 
characters cannot be questioned, nor that deficiency in such 
matters may be fatal to the individual. The matter may be 


summed up in a few words. It is the fittest that survive in 
the struggle for existence. 

819. Natural Selection. In this brood of rabbits we have 
imagined a process of natural selection to take place by which 
the unfit are eliminated. Since the parent rabbits succeeded 
they must have been fit and, therefore, the young ones which 
resembled the parents closely would be fit also, provided they 
lived under the same conditions. This process would, there- 
fore, tend to preserve the type of the parents. But conditions 
change and a locality which at one time is most favorable for 
a given species may become less so. Moreover, species often 
migrate. If a locality becomes overcrowded or the food 
scarce, or enemies too numerous, there is a special impetus 
given to the migrating tendency. Thus a species may push 
out into a new and quite different environment where there is 
a different nature at the work of selecting. Suppose again 
the rabbits: They have pushed out into colder regions, where 
the snow lies on the ground for many months. The gray rabbit 
would be very conspicuous against the snow and a coat of white 
fur would be decidedly advantageous in winter. Hence in- 
dividuals with white coats in winter might be selected here, 
while in other regions the winter gray continues to hold the 
advantage. The diverse conditions would thus tend to pro- 
duce two varieties of rabbit or indeed two species, the winter 
white and the winter gray. So long as the two kinds remain 
connected by intermediate forms they could be only called 
varieties, but if the intermediate forms disappear they would 
be distinct species. This is not intended to be regarded as 
an explanation of how the two species of rabbits originated. 
It is simply a hypothetical case which may help one to an under- 
standing of the method by which natural selection, acting 
through individual variation, may produce new species. The 
process of natural selection must necessarily be a very slow 
one, and there are few historical records of such changes. 


There is, however, much indirect evidence, which we will now 

820. Animals and Plants Under Domestication. All our 
domestic animals and plants were originally wild species. They 
have become so changed under domestication that most of them 
bear little resemblance to their prototypes. The various 
types of fancy pigeons, the carriers, fantail, tumbler, pouter, 
etc., have probably all been produced by selective breeding 
from'the wild rock pigeon. The differences in structure of these 
fancy pigeons is much more than enough to give them specific 
standing. Such differences found in wild species would be 
regarded as of generic value. How did they come about? 
Simply by selection. The breeder selects those which conform 
to a certain type and thus produces a "breed." 

821. Our dogs may be descended from two or three wild dogs 
or wolves, but the original type has little in common with the 
hundreds of breeds of dogs, ranging from mastiff to greyhound 
and from poodle to St. Bernard. In our horses and cattle, cats, 
poultry, garden vegetables and cereals similar remarkable 
effects have been produced. In some of these domestic breeds 
selection has been at work for a long period, but often marked 
results have been brought about in a short time. 

822. The conclusion which we may draw from the facts of 
varieties under domestication is that species are not immutable, 
and if man by selection can produce such results it is reasonable 
to believe that nature by some process may bring about similar 
results. The natural process is certainly slower, but the time 
during which it has been at work is vastly longer. 

823. Geographical Distribution. If such species had an 
independent origin (not by descent) then there is no reason 
why two similar species should be related geographically. 
They may as well occupy islands on the opposite sides of the 
globe, provided food, climate, etc., are the same, as live in adja- 
cent countries. If, however, two species had a common origin 


(by descent) they must also be related geographically; either 
they still live in the land where they originated or else there 
must have been a path along which they could migrate to the 
place where they are now found. 

824. Hence, the present geographical distribution of animals 
may throw light on the question of the origin of species. 

825. Australia is separated from Asia by a barrier which is 
practically impassable for mammals, and geologists tell us that 
this has been the case for a long time since the Cretaceous 
period. Now if species change in the course of time one would 
expect to find the Australian mammals unlike those of Asia or 
elsewhere. This expectation is fulfilled in a remarkable way. 
All the mammals, except a few which we have reason to believe 
were carried there, are Monotremes and Marsupials. These 
are the most primitive mammals and no living forms are found 
outside of the Australian region except the American opossum. 
Fossil remains show that Marsupials were at one time wide- 
spread, but evidently they were unable to contend with the 
higher mammals and became extinct. In Australia no higher 
mammals developed. On the other hand the Marsupials 
developed in great variety; herbivores, carnivores, gnawers, 
subterranean mole-like forms and tree-dwelling forms. 

826. Africa and South America are also somewhat isolated, 
and here we also find peculiar faunas. It is not remarkable 
that a peculiar species of any kind should be confined to a given 
area, but where several similar species of a remarkable genus are 
found in the same isolated region, and when, farther, fossils of 
still other related forms are found in the same region, only the 
hypothesis of a common origin offers a satisfactory explanation. 
Numerous examples of this kind occur. Some examples are 
the following: The kiwi-kiwis of New Zealand, the catarrhine 
monkeys of the Old World and the platyrhine monkeys of the 
New World, and the rheas of South America. The Edentates 
of the Old and New Worlds are also of distinct orders. 


827. The fauna of isolated oceanic islands contributes the 
same kind of evidence. Madagascar has many species found 
nowhere else, but these species are more like those of the 
neighboring African coast than those of more remote regions. 
The Galapagos Islands also have a peculiar fauna, which finds 
its greatest affinity in the fauna of the nearest part of the South 
American coast. The fauna of the Azores is related to that of 
Europe, while that of the Bermudas belongs to America. 
These resemblances of faunas cannot be attributed to the effect 
of food and climate or other external causes, for there is often 
a greater difference in environment between two neighboring 
localities than between others on opposite sides of the globe. 

828. The Hawaiian Islands are very mountainous and the 
mountains are cut by numerous deep parallel valleys. In 
these valleys are found numerous species of the snail, Achati- 
nella. The snails cannot cross the mountain barriers and hence 
there is little migrating of snails from valley to valley. The 
fact that almost every valley has its own peculiar type of 
snail, and the way these species are distributed on the island, 
makes it seem probable that all had a common origin and 
that each species originated where it is now found. If at any 
time, in any valley, a new character appears through individual 
variation that character may in time be transmitted to all the 
individuals of that valley, and hence become a specific char- 
acter, but natural barriers will prevent its transmission to other 
species living in other valleys. 


829. In Part I many examples of modification of the type 
structures were described and it was shown that these modifi- 
cations were always associated with peculiarities of life habit 
or of environment. Such modifications of an organism, in 
connection with peculiarities of the condition of life, are known 


as adaptations. It is a phenomenon not confined to plants. 
The peculiarities of parasitic animals or of sedentary animals 
are also adaptations. In fact, the sum total of characteristics 
of living things is an adaptation to conditions. At various 
points in this book the idea has been expressed that an organ- 
ism is what it is because of the conditions under which it lives. 
For example, one often hears stated that foliage is green because 
that color is least irritating to the human eye. The same idea 
is expressed in other forms, but the position or point of view is 
entirely false. The color of vegetation is part of the environ- 
ment, and natural selection would result in the development of 
eyes which were adapted to that color. 

830. There are many special types of adaptations which 
are of interest because they give us an insight into the method 
of operation of natural selection. We will call attention to a 
few of these here. 

831. The dispersal larvae so often found among marine ani- 
mals may be regarded as an adaptation by which the species 
make use of oceanic currents to secure transportation from 
place to place. The various devices employed by plants to 
secure the scattering of their seeds fall in the same category. 
On the other hand a free-swimming larva would be fatal to 
many fresh-water animals because the larvae would be carried 
to the sea and perish in the salt water. The eggs of very 
many marine fishes are light and float freely in the water, but 
the eggs of fresh-water fishes are either attached by means of 
adhesive secretions or else are so heavy that they lie on the 
bottom among the pebbles which protect them from the cur- 
rent. Many fresh-water fishes make nests by excavating the 
bottom and the eggs are often covered by a layer of pebbles. 
Some marine fishes also attach their eggs or construct nests, but 
the habit is not general. 

832. The eggs of the decapod Crustaceae are usually attached 
to the abdomen of the female, but in the marine forms the 


young hatch early and are then set free, while the young of the 
crayfish hatch later and cling to the mother for some time after 

833. The Amphibia generally deposit their eggs in water, 
but they avoid the streams which have a strong current, 
preferring quiet pools, ponds or even stagnant puddles. They 
also often attach the eggs to objects under water by means of 
the gelatinous envelope^ which holds the eggs together in masses. 

834. The embryo of the marine Lamellibranchs is set free 
at an early stage as a free-swimming veliger (page 338). At a 
corresponding stage the young glochidium of the fresh-water 
mussels, the Unios and Anodontas, become attached to fishes, 
where they continue their development for perhaps several 
months longer before they finally become free. 

835. Lakes and ponds often swarm with many kinds of 
minute free-swimming organisms, which are comparatively 
rare in streams. Only the larger forms, with their stronger 
swimming powers, can make headway against an ordinary 
current, and are thereby enabled to maintain themselves in 
the waters of creeks and rivers. 

836. The insect faunas of oceanic islands present a similar 
phenomenon. These insects are either wingless or, if they 
have wings, are seldom seen to use them. The explanation 
offered is very simple. Few insects are able to fly against a 
strong wind, and strong winds are particularly prevalent on 
oceanic islands. Under such circumstances if an insect were 
to rise into the air it would most likely be carried to sea 
and perish. As a result only those insects which cannot, or at 
least do not, fly have remained. 

837. Adaptations to Water. Attention may again be called 
to the important adaptations which have reference to water. 
These are particularly well exemplified by comparing Hydro- 
phytes and Xerophytes, or by comparing aquatic and terrestrial 
animals, especially with regard to the integument. 


838. Adaptations with regard to light are much more gen- 
eral and important among plants than among animals. As 
touching plants the matter has already been fully discussed. 
Animals are much less dependent on light, and adaptation with 
regard to light affects chiefly the eyes. Animals adapted to 
absolute darkness, such as the fishes, salamanders and cray- 
fishes of caves have usually little or no pigment in the skin. 
The most striking peculiarity of these animals is that they are 
blind and the eyes are usually very rudimentary. The tactile 
organs, however, are better developed than are those of the 
normal type. This is especially true of the antennae of the 
crayfishes. Blind fishes are found in caves in many parts of 
the world, and they "belong to many different families, but are 
always related (similar) to the forms living in nearby streams." 
This fact is strong evidence that the blind forms have descended 
from the ancestors of those which now live in the surface 

839. Adaptations to Changes of Temperature. All plants 
and most animals are directly dependent on the temperature 
of the surrounding medium, so that growth and other vital 
processes proceed more or less rapidly in accordance with the 
changes in temperature of the surrounding water or air. The 
time required for a frog to develop from the egg, for example, 
may vary from seventy days at a temperature of 60 F. to two 
hundred and thirty days at 51 F. The temperature of sea 
water seldom passes the limits within which vital processes 
are possible. The temperature of the air varies much more 
widely, and consequently terrestrial organisms present several 
types of special adaptations with regard to temperature. All 
the vital activities of all terrestrial plants and animals, except 
birds and mammals, are suspended when the temperature falls 
to or below freezing. Insects, Amphibia and Reptiles usually 
hide in sheltered recesses at such times and remain dormant until 


the temperature rises again to a point which will permit the 
organs to perform their functions. 

840. Birds and Mammals present a special adaptation in 
this regard in the comparatively high and constant temperature 
maintained by the body. This is done by the expenditure of a 
considerable amount of energy, specifically for keeping the 
body warm, and for this reason Birds and Mammals require 
considerably more food than do other animals of corresponding 
size. The temperature of the body is kept constant by a 
control mechanism by which the amount of heat lost is con- 
trolled. Much heat is lost by evaporation of moisture from 
the lungs and respiratory passages and from the mouth cavity 
also when the animal is panting. Panting greatly increases 
the amount of vaporization and the accompanying loss of heat. 
The feathers and hair, when lying close, prevent the loss of 
heat, but they may be raised on end by the action of special 
muscles in the skin. This permits free circulation of air and 
increases the loss of heat by radiation, convection and vaporiza- 
tion. The skin of Mammals is provided with numerous tubu- 
lar glands which discharge their secretion on the surface. This 
secretion is chiefly water, which evaporates from the surface as 
rapidly as it is formed, or may accumulate in small droplets of 
perspiration. The function of the sweat glands seems to be 
primarily temperature control. 

841. The heat of the body is distributed by the blood. The 
amount of blood brought to the surface, and there cooled, is 
regulated by the expansion and contraction of the blood vessels 
of the skin. The supply of blood to the skin and the activ- 
ity of the sweat glands are both controlled through nervous 

842. Some Mammals have in a measure reverted to the primi- 
tive condition of variable temperature. Bears and many 
others pass a considerable portion of the winter in a deep sleep, 
during which the temperature of the body falls to a low point 


and all the vital processes are at a low ebb. This sleep is 
called hibernation. It permits the animal to tide over the sea- 
son when food is difficult to find. The small amount of energy 
required to maintain life in the hibernating condition is fur- 
nished by the reserve store in the form of fat which the animal 
possesses at the beginning of winter. 

843. Most Birds have adapted themselves to the changing 
seasons in another way. At the approach of the winter season 
they move southward in easy flights of twenty-five to fifty 
miles a day and thus keep south of the region of ice. In the 
spring this migration is repeated in the opposite direction. We 
do not know what impels the birds to begin their migration, 
for they do not wait until the season has advanced far enough 
to make conditions uncomfortable. Nor do we know by what 
means the bird is informed in which direction to fly. We call 
such actions instinctive, which, however, does not explain them. 
It may be that they should be classed with such rhythmical 
physiological processes as the fall of the leaves of deciduous 
trees, and tropisms like geotropism and heliotropism. But 
whether the fact is explained or not the real fact remains, 
and if a bird failed to migrate, that bird would probably not 
survive the winter and its eccentricities would not be 

844. Adaptations for securing food are exceedingly manifold. 
Under this class would fall most of the peculiarities connected 
with saprophy tic and parasitic habits. Insectivorous plants, the 
teeth and digestive tract of the carnivorous and herbivorous 
animals, the claws, beak and digestive tract of the birds of prey, 
and the digestive tract of grammivorous birds may be cited 
in this connection. The great baleen whale feeds on minute 
pelagic organisms, which it secures by filling its mouth with the 
water containing the food and then straining out the food by 
allowing the water to flow out through the fringe of horny 
baleen fibres which hangs down from the upper jaw. Many 



spiders spin a web which entangles small insects which come 

in contact with it. Many other animals set traps. Some 

aquatic insect larvae construct a 

trap like a fish net and these open 

up stream, so that the current may 

sweep small edible objects into 

them. The angler fish lies on the 

bottom, of ten very much concealed. 

His most capacious mouth opens 

upward and above it hangs an 

attractive-looking bait, which is 

phosphorescent in some cases. 

When the prey approaches near 

enough the great mouth opens 

suddenly and the rush of water 

into it carries the prey along. An 

African snake (Dasy-peltis) feeds 

largely on eggs,which are swallowed 

whole. Some of the vertebrae have 

pointed processes which project 

into the oesophagus. The shells of 

the eggs are broken against these 

points and the empty shell is then 


845. Pollination. Some of the 
adaptations of plants, with regard 
to pollination, have been noted 
elsewhere, but there are many 
special cases which are very re- 
markable. We can only note 
briefly a few of them. 

846. The papilionaceous bios- which the f 00 d is strained out of 
som of most members of the the water - 

pea family is familiar to everyone. This flower in its prime 

FIG. 250. Part of one of the 
horny baleen plates (" whalebone ") 
which hang from the upper jaw of 
the baleen whale. One edge of the 
plate is split up into a fringe of 
hairs which form the filter through 


has its axis horizontal. The upper petal (standard) is very 
broad and stands erect, making the flower very conspicuous. 
The two lateral petals (wings) form a horizontal platform upon 

FIG. 251. A row of the nets woven by the Caddice fly larvae, to catch food. 
In the next figure three of the nets are seen from above. 

which an insect (bee) may conveniently rest when visiting the 
flower for nectar. The two lower petals are slightly curled longi- 
tudinally and have their concave faces toward each other (keel) , 
so that they completely enclose the stamens and pistil. The 
filaments of the ten stamens are all united, except the upper 

FIG. 252. Nets of Caddice worm. See Fig. 251. 

one, for most of their lengths. The ends only are free and 
bend upward. The style also bends upward at the end. 
When an insect like a bee visits the flower its weight presses the 



wings and keel down, but the rigid filament tube holds the stamens 
and style in place. The upper edges of the keel petals separate 
and the anthers come into view and touch the ventral surface 
of the abdomen of the insect, leaving upon it a charge of pollen. 
The insect now visits another flower and takes the same posi- 

FIG. 253. Bumblebee (Brombus) pushing his way under the stigma and stamen 
of the blue flag (Iris versicolor). See following figure. (From Folsom.) 

tion on the wings. The stigma touches the same part of the 
insect which before came in contact with the stamens and is 
consequently covered with pollen which came from another 

847. In some of the mint family (Salvia) the flowers are 
somewhat funnel-shaped, with two stamens attached to the 



corolla near the opening of the funnel. Each anther is at- 
tached to the upper end of a long lever, which is pivoted in the 
middle. When an insect enters the flower it brushes against 
the lower end of this lever, causing it to rise, and the opposite 
end with the anther comes down on the insect's back, leaving 

FIG. 254. Diagram to explain the preceding figure. The bee alights on the 
spreading lobe of the perianth (s) and forces his way under the stigmatic shelf 
(/) and the anther (an). In doing so some of the pollen left on his back from a 
flower previously visited, is scraped off by the stigma and then he is immediately 
dusted with pollen again by contact with the anther. The nectary is shown 
at n. 

a dab of pollen upon it. At this time the stigma is high up 
under the hood-like edge of the corolla, but later it grows out 
and down so that it assumes approximately the place where the 
anther strikes the insect. When now a bee which has previously 
been dusted with pollen visits this flower the stigma brushes 
against its body and is pollinated. 


848. Some of the orchids present the most remarkable 
adaptations for pollination through the agency of insects. 
In Arethusa the pollen is contained in a receptacle which 
opens by a lid. This lid is torn open by the insect in its 
efforts to back out of the flower, and the pollen falls upon its 
back. In backing out of the flower, however, the insect first 
brushes against the stigma, which would then be pollinated, 
provided the insect had previously visited another similar 

849. The little showy orchid (Galeorchis) has two pollen masses 
(pollinia) which lie in the throat of the corolla, one on either 
side of the stigma. Each pollen mass consists of pollen grains 
bound together by threatts and is attached to a sort of stalk 
which ends in a viscid disc. The polliriium is enclosed in a sack, 
but the disc is exposed and projects forward toward the entrance 

o the corolla. When an insect thrusts its head into the throat 
of the corolla, as it must in order to reach into the deep nectary, 
the discs of the pollinia adhere to the eyes or some other part 
of the head and are withdrawn when the insect leaves the 
flower. The position of the pollinia is now such that when 
another similar flower is visited by the insect the pollinia are 
thrust directly upon the broad stigmatic surface. 

850. The lady slipper (Cyprepedium) has again another 
device. The large cup formed by the "lip" of the corolla is 
readily entered, but exit is difficult because of the way the edges 
are inrolled. A small opening on either side of the column, 
which bears the two anthers and the stigma, attracts the atten- 
tion of the prisoner and he forces his way through one of these. 
In doing so he must creep under the column and his back 
brushes against the stigma first and then the anther. If he 
had previously visited a similar flower some of the pollen on 
his back would now adhere to the stigma and^a new supply 
of pollen would immediately be obtained as he passes the anther, 
for the next flower visited. 




851. The flower of the common milkweed (Asclepias) is 
greatly modified. The five stamens are all united around the 
ovary and they alternate with five funnel-shaped nectaries. 
Externally nothing can be seen of either anthers or stigmas, 
but alternating with the five nectaries are five narrow slits 
which open slightly at the upper end of the pendant flowers. 
These slits open into the stigmatic cavities. The pollen is not 
powdery but adheres in masses similar to the pollinia of Galeor- 


FIG. 255. Structure of the milkweed flower. (Figures A and B should be 
inverted.) A, The whole flower; B, the upper part of A enlarged; C, corolla; 
/, slit opening into the stigmatic cavity; h, nectary. C, a pair of pollen masses 
connected by a V-shaped appendage and the sticky disc (d). (From Folsom.) 

chis. There are five pairs of such pollen masses and the pairs are 
united in the form of a letter V, with a sticky substance at the 
point of the V. The apex of the V coincides with the lower end 
of the slit and the pollen masses are embedded in pockets which 
extend upward on either side of the stigmatic cavity. When the 
bee is clinging "head down'' to the pendant flower his feet 
readily slip into the slits and are thereby guided to the sticky 
apex of the V pollen mass. With a strong pull the foot is re- 
leased, with the pollen masses attached. When later the same 



foot slips into a similar slit the pollen masses are drawn into the 
stigmatic cavity, and in part or wholly torn off by the struggle 
of the insect. Occasionally an insect is not strong enough to 
free itself from these traps and perishes, suspended by one or 
several feet. This is the only possible method of pollination in 

852. The flowers of the Yucca are pollinated by the Pronuba 
moth. The moth deliberately collects the pollen with the fore 
feet, then goes to the pistil of the same flower, pierces it with 
her ovipositor and deposits an egg. She then goes to the fun- 

FIG. 256. A wasp, Sphex ichneumonea, with a number of the milk weed pollen 
masses attached to its feet. 

nel-like stigma and deposits the pollen. This operation is re- 
peated in the same and neighboring flowers until her eggs are all 
deposited. The carrying of pollen to the stigma is an indirect 
method of providing, food for the young, since it causes, the 
ovules to develop, and these are devoured by the larvae of the 
moth after hatching. When ready to pupate the larvae escape 
from the ovary by a hole which they bore through its wall. 
Not all the developing seeds are devoured. The plant sacri- 
fices a part of the seeds as a reward for the services of 

853. It is not only the plants that are affected by the adapta- 



tion for pollination. The insects are often as much modified. 
The worker bee has special organs for carrying pollen, the Moth 
has a long proboscis for reaching into the extremely deep necta- 
ries of certain flowers and thePronubahas developed a special in- 
stinct for pollinating the Yucca, which is as much a part of the 
insect as are its legs and wings. In many cases both plant and 
flower are so modified with respect to each other that one can- 
not exist without the other. 

FIG. 257. FIG. 258. 

FIG. 257. Pronuba yuccasella in the flower of the Yucca. (From Folsom 
after Riley.) 

FIG. 258. Female Pronuba getting pollen from the anther of Yucca. (From 
Folsom after Riley.) 

854. Care of Young. The food stored up in seeds and in eggs 
is not the only kind of provision made for the young of the next 
generation. Birds feed the young until they are able to hunt 
food for themselves, and the young of Mammals are fed for some 
time from the secretions of the mammary glands of the female. 
But many more special adaptations occur. To mention only 
one from the plants: The mangrove trees grow in shallow 
water, but the seeds are not allowed to fall and drown or be 



FIG. 259. The developing seedling of the mangrove. In i, 2 and 3, the 
fruit is still hanging on the branch but the hypocotyl grows until it reaches a 
length of about 1 2 inches when it drops to the earth and, striking in the mud, 
remains standing upright, as if planted (4)- At a later stage (5) roots have been 
developed at the lower end and an epicotyl from the upper end. 


covered up with the mud or washed away. The seed remains 
on the tree until it has germinated and developed a heavy 
hypocotyl about a foot in length. It then falls and strikes deep 
enough into the mud so that it remains upright as if planted. 

855. Among the social Hymenoptera the young are cared for 
as carefully as they are by the higher vertebrates. In other 
cases, however, the young are never seen by the mother, and, 
indeed, in many cases, the mother dies before they are hatched. 
But even in such cases the mother may make elaborate provision 
for the young. The Pronuba will illustrate this point, but 
another more remarkable example is frequently quoted. Many 
of the solitary wasps (Sphegidae) excavate channels under ground 
or build mud nests under the eaves of houses. These nests are 
then filled with spiders or other insects which the wasp stings in 
such a fashion that they are paralyzed, but not killed. An egg 
is then deposited by the wasp and the nest sealed up. The 
larva soon hatches and feeds upon the spiders. If the spiders 
had been killed decomposition would soon set in and the result 
would probably be the death of the wasp larva. If the spiders 
were not sufficiently stupefied they would probably kill the larva. 
It is, therefore, of great importance that the spiders should be 
stung in a very particular manner. But the wasp never returns 
to the nest and cannot know how it fares with her offspring. 
If, however, her work was not well done she will have no off- 
spring to inherit her careless ways. Our wonder and admira- 
tion of the instinct and skill of the successful wasp are increased 
when we consider that the proper stinging of the spiders is not a 
deed that is performed with calm deliberation, for the spiders 
are also armed and are bold and skillful fighters. The wasp is 
compelled to place carefully the paralyzing thrust in the midst of 
a desperate conflict. 

856. Sexual Dimorphism. Reference has been made to 
sexual dimorphism (page 353). This is more general among 
the higher animals. When there is a notable difference between 



the males and females of a species of one of the lower phyla 
the difference is most frequently a difference of size. The 
female is usually larger. This is regarded as due to the great 
demands made upon the female organism in the development 
of the relatively large mass of egg substance. Among Mammals 
the male is usually the larger, if there is any 
marked difference in size. The males of the 
fur seal and sea lion are about four times as 
large as the female. In such cases the males 
fight for the possession of the females, and 
consequently size and strength are the deter- 
mining factors in the struggle for existence 
among the males. Why the size of the male 
parent should be inherited by the male off- 
spring and not by the female is an interesting 
problem which still awaits solution. 

857. Sexual Selection. In some species of 
Birds also the males are the larger, and this 
occurs again in those cases in which the males 
contend in battle for the possession of the 
females. Generally, however, the male bird is 
distinguished from the female by his greater FIG. 260. 

i T . . * !, v , Oueen of Termes 

beauty or his superior ability as a vocalist, obesus. Natural 
This introduces us to a particular form of size - (From 
natural selection, called sexual selection. At 
the time of mating the males vie with each 
other by displaying their beautiful plumage or 
singing their best songs. This is done in the 
presence of the female, and is evidently intended 
to win her favor. Following this courtship the birds mate in 
pairs (monogamy), apparently according as the appearance or 
performance of the male pleases the female. That is to say, 
in this case the selecting which determines the male parentage 
of the next generation is done by the female, and the survival 

size . 
F o 1 s o m after 
Hagen). It is 
said that in some 
cases the queen 
attains the size 
of thirty thou- 
sand workers. 



of the male characters in the next generation depends upon 
the colors, plumage or song, which are fitted to meet with the 
approval of the female. Sexual selection also occurs in other 

FIG. 261. Myrrh, a xerophytic plant of the Arabian desert. There are few 
leaves and the spiny branches offer protection against grazing animals. (From 

classes of animals. A particularly interesting case is found 
among spiders. Here the males are often much smaller than 
the female, and strive to gain her favor by series of movements 
which may be called dancing. 



858. Welfare of the Individual and of the Species. The 

welfare of a species sometimes makes demands which do not 
coincide with what is required for the welfare of the individuals 
of that species. It is sometimes in the interest of the species 
that certain individuals should perish. It is often better that 
the weak or inefficient individuals should perish, and in many 

FIG. 262. A sandy beach, the home of a small light-gray grasshopper. The 
dark patches in the background are green yaupon thickets where two species of 
green grasshoppers are found. These are remarkably well protected by their 
coloration when in their proper environment. Coast of North Carolina. 

cases the welfare of the species requires the sacrifice of the 
most efficient individuals. In many instances the female 
dies immediately after she has properly deposited her brood of 
eggs. This is not only because her life term has been completed 
but because of the heavy draught made upon her organism 
by the development of the ova. Sometimes the body of the 


female disintegrates normally in order to free the contained 

859. Though such cases are rather numerous they must 
still be regarded as exceptional. Usually the best interests 
of the species are served by that which favors the individual. 
We will, therefore, inquire into that class of adaptation by 
which the individual profits more directly. Much of the 
struggle for existence is in reality a struggle between indi- 
viduals; it may be individuals of the same or of different 

FIG. 263. A Juncus swamp in which is found a large gray and olive-brown 
striped grasshopper. The color of the grasshopper is protective in such an 
environment. Coast of North Carolina. 

species. Illustrations of thi? principle are usually taken 
chiefly from animals, but one from the vegetable kingdom 
may also be introduced here. The difficult conditions under 
which desert plants grow makes the number of individual 
plants which succeed relatively small. Therefore, the life 
of an individual plant is of more value to the species. It is, 
therefore, not strange that such plants are wonderfully well 
protected. The spines of the cacti render them practically 
immune to the attacks of animals. To a less degree spines 



are also found on mescphytic vegetation and are also more or 
less efficient as protection. Many plants are protected by 
bitter, acrid, poisonous or otherwise disagreeable juices which 
protect them from the attacks of many, if not all, animals. 
When we consider how devastating the attacks of insects 

FIG. 264. Protective resemblance of the moth, Catocala lacrymosa, to the 
bark on which it rests. A, With wings spread, as in flight; B, with wings folded 
and at rest on bark. (From Folsom.) 

sometimes become to given species of plants we may realize 
how useful such protective contrivances may be. 

860. Animal Coloration. If the color of animals has no 
general significance it does have an extremely far-reaching 



significance when we consider the life of the individual. The 
color is often the most remarkable adaptive feature about it. 
Animals are usually darker above than below. This may be 
due in part to the direct tendency of light to produce pigment 
in the skin. But it has also been suggested that the dark 
upper surface in the bright light and the light under surface 

FIG. 265. Walking-stick insect (Diapheromera). Natural size. An example 
of protective resemblance. (From Galloway after Folsom.) 

in the shadow of the body yield approximately equal lighl 
value and tend to render the animal inconspicuous at a distance 
86 1. Animals generally are colored in harmony with theii 
surroundings. Polar animals are white; animals of the deserl 
and plain are gray, and those of the forest are striped or mot- 
tled. Pelagic marine animals are often transparent and those 
of the bottom are often so much like the bottom that it be- 


comes difficult to distinguish them from their surroundings, 
even when one knows precisely their location. Many animals 
living on or among green foliage are as green as the leaves. 
These agreements in color between the animal and its environ- 
ment render the animal difficult to see and, therefore, protect 
it from its enemies. The same characteristic of the animal 
enables it to steal upon its prey, but in either case the color is 
an advantage and may have been developed through natural 

862. It has been shown that some color patterns, which at 
first sight seem to render the wearer conspicuous, have in 

FIG. 266. Protective resemblance. A sea-horse resembling a sea-weed. 
(From Galloway after Eckstein.) 

reality an obliterating effect when seen at a distance under 
natural surroundings. Nevertheless, there are many cases 
in which the color makes the animal conspicuous. This may 
be illustrated by the many species of birds in which the 
male is brilliantly colored. An explanation for this coloring 
has already been given. But the females of these same species 
are usually very plainly colored and harmonize well with their 
surroundings. The female usually broods over the eggs, and 




FIG. 267. The mimicry of Papilio merope. 

A, Papilio merope (male). 

B, Papilio merope, female; mimics Amauris echeria. 

C, Papilio merope, female; mimics Danais chrysippus. 

D, Papilio merope, female; mimics Amauris niavius. 


when sitting quietly is not readily seen. She is, therefore, 
probably less frequently molested. The male is free to take 
to flight when discovered and pursued. The color of the female 
is explained on the basis of ordinary natural selection, while 
that of the male is due to the operation of sexual selection. 

863. Protective Resemblance. In many cases the animal 
resembles its environment in form as well as color. This is 
called protective resemblance. There are insects which 
resemble dead twigs, rolled and broken dry leaves, dead 
leaves still on the twig, green leaves, seed pods, patches of 
lichens, etc. There are fishes which resemble sea weeds and 
even a mammal, the sloth, resembles a lichen-covered knot on a 

864. Feigning. Many animals when threatened by enemies 
resort to bluff. They assume terrifying attitudes, make a show 
of great size by swelling themselves or raising hair or feathers 
on end, or make disconcerting noises like hissing, spitting or 
growling. Feigning death or "possuming" is another com- 
mon instinctive method of getting out of a tight place. The 
opossum is a well-known example and has lent his name to 
this particular instinct. Beetles often feign death. When 
attacked they allow themselves to fall to the ground and lie 
there motionless for some time. They are then difficult to 
find, whereas if they attempted to run or fly their move- 

The three types of female Papilios, shown on the left in B, C, and D, belong to 
the same species, of which the male is represented in A . There is also a type of 
female which resembles the male, and still another form which is not figured 
here. There are then five types of females within this species. Three of these, 
beside the male, have been reared from the same brood of eggs. This species 
is found in Africa but a similar case of polymorphism is found in India. The 
origin of the polymorphism in this case is apparently due to mimicry. The 
species of Amauris and Danais represented on the right in the figure are pro- 
tected, i.e., they are unpalatable to birds, hence the female Papilios by mimicry 
also secure immunity from the attacks of birds though they are not otherwise 
protected. The males are not protected, nor are they mimics, but they are pro- 
duced in much greater number than the females. In A the predominant color 
is yellow, in C it is orange, while in B and D it is white, or pale yellow, and dark 
gray to black. Xi/2. 


ments would make them conspicuous and invite a second 

865. Mimicry. Many animals are protected in various 
degree by their stings, poison fangs, malodorous secretions 
or unpalatable taste. These are naturally avoided by species 
which would otherwise prey upon them. But this means that 
the preying species must be able to distinguish between the 
palatable and unpalatable prey. The unpalatable forms are 
often conspicuously marked, as if to advertise the fact, and 
thus prevent an attack which might be disastrous to both 
pursuer and pursued. Coloring which is regarded as serving 
such an end is termed warning coloration. It is especially 
common among insects and protects them from birds. 

866. Where species occur which are protected and warn- 
ingly colored there are also often other species which are not 
protected by sting or unpalatable, and yet are very like the 
protected species in form and color. This is called " mimicry," 
because the one form is supposed to have acquired a resem- 
blance to the other for the purpose of protection. If a bird 
recognizes a certain form and color pattern as belonging to an 
undesirable insect another insect resembling the first would be 
spared an attack if the bird failed to discriminate. Upon this 
ground mimicry is explained by natural selection. A fact of 
almost conclusive significance is that the mimic and the model 
are always found associated in the same regions. Mimicry 
is very common among butterflies, but many cases are known 
in which bees, bumbleebees and wasps serve as models and are 
mimicked by flies, beetles and butterflies. Cases in which 
poisonous snakes are mimicked by harmless ones are also known. 

867. Mimicry also occurs between two species which are 
both protected. This demands a different explanation from 
the preceding case. Birds learn that protected species are 
unpalatable only by experience, and in getting this knowledge, 
one or more butterflies of the protected species are injured or 


destroyed. Therefore, each species profits by the sacrifice 
the other makes in the education of the bird. In the case of 
the unprotected mimic, however, the burden of education rests 
wholly on the protected model. 

868. Color Changes. The color of animals is in the main 
inherited, but it may often be more or less modified in the 
individual by the environment. Animals kept in darkness 
tend to become paler, and if the young flat fishes are illuminated 
from below there is a tendency for the underside to develop 
pigment where normally there is none. The color of cater- 
pillars may be determined somewhat by the color of the sur- 
rounding objects. If they are surrounded by dark objects 
they tend to become darker also. Other animals, like many 
fishes, frogs, tree toads, lizards and cuttlefishes, change color 
rapidly and repeatedly. The color of a cuttlefish changes in a 
flash. This is due to the action of contractile pigment cells. 
When the cells expand the animal takes on the color of the 
chromatophores, dark brown or orange or a combination of 
these colors, as the different cells are stimulated. The action 
is controlled through the nervous system, but it is not clear 
what service it plays in the animal economy. The chameleon 
and other lizards, as well as many frogs and fishes, change 
color more slowly, but by a similar mechanism. In these 
cases the color assumed is determined by the surroundings. 
If the animal is blind the changes do not occur, and it is known 
that the stimulus is received through the eye and transmitted 
by the nervous system without, however, any voluntary 
control by the animal. In these cases the colors assumed are 

869. Luminescence. There are two important physiological 
processes which are practically wanting in all animals above 
fishes, but are quite common among lower forms. These are 
the light-producing organs and organs for generating electricity. 
The production of light or phosphorescence occurs among all 



classes of animals from the Protozoa to Fishes, and also among 
fungi. Decaying vegetable and animal matter is often lumi- 
nous as a 'result of the action of bacteria. The mycelium of 
other fungi also yields light at times. In these cases the light 
is apparently a by-product, which is not to be regarded as 
having any adaptive function. 

870. The light-producing power is especially common 
among marine invertebrates and Fishes. Phosphorescence 
occurs among the Protozoa, Coelenterates, Worms, the smaller 
Crustaceae, Bryozoa, Rotifers, free-swimming Ascidians, Fishes 
and Insects. In some cases the light is produced at certain 
points within the protoplasm of the cell. In others the slime 
secreted by glands in the skin is luminous. And again there 
are special organs which may be simple or complex in structure, 
but which bear evidence of having been developed from groups 
of glands, though there is no duct and no external secretion. 
The more highly developed photogenic organs have a structure 
resembling that of an eye with a layer of pigment at the back 
and a lens in front. In the lower animals the light is only 
emitted when the animal is stimulated, and the purpose of it is 
unknown. In the higher forms, however, the organ is well 
supplied with nerve elements and is under voluntary nervous 

871. Among Fishes the photogenic organs are especially 
common and well developed in deep sea species. Some " an- 
glers" carry a lantern at the tip of the long anterior dorsal 
spine. This lantern is suspended directly above the mouth 
of the animal and is regarded as a bait by which the angler 
attracts his prey. In other cases the photogenic organs may 
occur on almost any part of the body. 

872. The common "fire fly" is a beetle belonging to the 
family, "Lampyridae," which contains many luminescent 
species. The eggs, larvae, male and female, may all be lumi- 
nescent. In some species the female is wingless, but has an 


exceptionally brilliant light, which may, therefore, enable the 
winged male to find her. In many cases the eyes of these 
nocturnal insects are unusually well developed, as is also the 
case with the deep sea fishes, a fact that renders probable the 
view that the luminescent organs are a means by which the 
sexes find each other. The luminescence of the eggs and larvae 
are not understood. Photogenic organs are common in other 
families of beetles, and also occur among flies. 

873. Oxygen is said to be necessary to the action of photo- 
genic organs, but no appreciable heat is generated. 

874. Electrical Organs. Organs for generating electricity 
are developed in a number of Fishes. The electric eel (Gym- 
notus) of the Amazon and Orinoco rivers, the electric catfish 
(Malapterurus) of tropical Africa, and the electric rays (Tor- 
pedo) of the warmer seas are all capable of producing an 
electrical discharge sufficient to stun large animals. The 
electric organ of the Gymnotus is a modified muscle of the 
ventral side of the long tail. In Torpedo the organ is also of 
modified muscles, but of the head region. In Malapterurus the 
glands of the skin have been the starting point from which 
the electric organ developed. In neither case, of course, 
does the fully developed electric organ bear any resemblance 
to muscle or gland. The nerves supplying the electric 
organs are developed to an extraordinary degree and the 
electric discharge is under voluntary control. These organs 
are doubtless organs of offense and defense. 

875. Instinct. When the Pronuba moth deposits her egg in the 
pistil of the Yucca and then stuffs pollen in the stigma she is not 
aware of the end secured by her acts. She does not know that 
pollination will cause the ovules to develop. She does not know 
that there are ovules. She cannot even know that she has de- 
posited an egg. She never sees the eggs; she never sees her off- 
spring. The whole performance is to her without meaning 
and is enacted in obedience to an internal impulse originat- 


ing in physiological processes connected with the organs of 

876. Practically the same may be said of the actions of the mud 
dauber when building her mud nest and filling it with embalmed 
spiders. Another example may be described. There is a fam- 
ily of beetles (Cantharidae) which are parasitic on other insects 
during the larval stages. In some cases (Sitaris) the eggs of the 
beetle are deposited on the ground in the vicinity of a bumble- 
bee's nest. A small active larva hatches from these eggs and 
this larva reaches the nest of the bumblebee in a very peculiar 
way. It does not seek the opening of the nest and thus make 
its way in, but waits until some living object like the bumblebee 
chances to come within reach, when it attaches itself to the legs 
or hairs of the body and is thus carried into the nest. The 
bumblebee builds a large cell of wax. This it fills with honey, 
and then deposits an egg on the honey and seals up the cell. 
At the moment when the egg is deposited the beetle larva 
attaches itself to the egg and is thus sealed up in the cell with 
the egg and honey. It first devours the egg, which requires al- 
most eight days' time. Then it undergoes a metamorphosis, 
after which it is adapted for feeding upon honey, which it could 
not do before. After about forty days' feeding on honey the 
supply is exhausted and the larva undergoes a second meta- 
morphosis. This is followed by several more metamorphoses, 
after which the adult beetle (blister beetle) appears. The 
notable thing about this life history is the means adopted by the 
minute larvae for reaching the nest of the bee. They will often 
attach themselves to other living objects, such as other insects 
or even a camel's hair brush. When they do this they perish, 
because they fail to reach the condition necessary for their 
future development. These larvae are not taught what they 
have to do to succeed; they cannot profit by the observation of 
others, and they cannot learn from experience. They are 
somehow impelled to attach themselves to other insects. If 


they are fortunate enough to attach themselves to the bee they 
succeed. Otherwise they perish. The female blister beetle is 
very prolific and deposits many thousands of eggs. Hence it is 
only necessary that a few of the thousands of larvae should 

877. Actions like those of the Pronuba, Sphex or blister beetle 
larva are called instinctive. They are not prompted by a kind 
of intelligence. Nor are they in any sense akin to intelligence, 
though among the higher animals it is often difficult to say 
whether an act is prompted by instinct or intelligence. 

878. If a moth habitually rests on surfaces which it resembles it 
does so instinctively, not because it has an intelligent compre- 
hension that it is thereby protected. When a caterpillar spins 
a cocoon it does so instinctively and not with the forethought of 
providing protection. When a young bird builds a nest it is 
impelled thereto by instinct, and the form and manner of build- 
ing the nest are also determined by instinct. In the latter case 
more or less evidence of intelligence may be discernible, but the 
process as a whole is instinctive. 

879. An instinct is a kind of adaptation and subject to the laws 
of heredity. Instinctive actions may, therefore, be developed 
under natural selection, just like other adaptive characters of 
an organism. 

880. Intelligence. Intelligence is found only among the most 
highly organized animals because it is dependent upon an 
efficient set of sense organs which yield accurate information 
concerning the environment, a flexible response mechanism 
which may react in multitudinous ways to the infinite 
variation in the conditions of existence, and an organ of con- 
trol, the brain. In practically all Birds and Mammals the first 
two of these conditions ^intelligence are well met, and yet there 
is a vast difference in intelligence within these classes. This is 
due to the difference in brain structure. The brain of the lowest 
Vertebrates is an inconceivably complex organ, and in the higher 


forms it is vastly more so. This organ enables the individual 
animal to profit by experience. A sensation or an experience 
of any kind to which the organism has once been subjected is in 
some way registered in the brain (memory) and through it the 
future responses are modified. The constant stream of highly 
complex stimuli which pour in upon the organsim from the en- 
vironment are sifted and analyzed in some way by the brain, 
and the appropriate responses determined (reason, judgment), 
and the proper motor stimuli sent out to the organs of response 
(will) . Animals guided by instinct inherit a few sets of more or 
less complex responses, which are set in motion by corresponding 
sets of stimuli, and these responses are little if at all modified. 
Responses prompted by intelligence are more variable as de- 
termined by variable external conditions, and the individual 
exercises an adaptive control. The brain might be called an 
organ of adaptation, for the degree in which the individual can 
adapt itself to its environment is a measure of its intelligence. 


Acarina, 571 

Accessory buds. 57 

Accipitres, 650 

Achatinella, distribution of, 828 

Achene, 156 

Acicula, a needle-shaped structure, 346 

Acrania, (Gk. having no skull), 624 

Actinomma, 126 

Adaptations, 829 

regarding food, 844 
light 838 

temperature, 839, 840 
water, 837 

Adelochorda, 617 

Adventitious buds, 57 

^Ecidiospores, 749 

yEpyornithes, 650 

African fauna, 826 

Agglutinin, 777 

Aggregate fruit, 164 

Air sacs, 647 

Albatross, 650 

Albumen, the white of an egg, 677 

Aleurone, 19, 95, 672 

Alexin, (Gk. to ward off), 776 

Alga, pi. algae (L. a sea weed). 

Algae, 177, 178 

blue-green, 232 

Allantois, one of the fcetal membranes 
of reptiles, birds and mam- 
mals, 174, 641, 651 

Alligator, 644 

Alternation of generations, 525, 741 

Altitude and plants, 204 

Ambulacral, pertaining to the system 
of tube feet of an echino- 

Ambulacral system, 550 

Amiatus, 635 

Amnion, one of the fcetal membranes 
of reptiles, birds and mam- 
mals, 641, 651 

Amoeba, 68, 178, 303 

chemical sense of, 353 
digestion in, 452, 453, 464 

Amceba, reproduction in, 495, 496 

respiration in, 476 

response in, 407 

sense of sight, 355 
of touch, 354 

temperature sense, 356 
Amcebina, 510 
Amphibia, 350, 637 

structure of, 637 
Amphineura, 593 
Amphioxus, syn. for Branchiostoma 


Amphipoda, 561, 563 
Amphitrite, 139 

Amphiuma, the "congo snake." 
Ampulla, 550 
Amylolytic, converting starch into 

sugar, 461 

Amylolytic ferments, 463 
Amyloplast a starch forming cor- 
puscle, 93, 672 
Amylopsin, 462 
Andreaceae, 277 
Andrcecium, 123, 128 
Anemophilous, pollinated by the wind, 

139, 140 

Angiosperms, 171, 298 
Angler fish, 844 
Animal coloration, 860 
Anisopoda, 561 

Annelida (L. annellus a ring) worms 
with a segmented, ringed 
body, 542 

Annelids, circulatory system of, 469, 

locomotion of, 410, 411 

nervous system of, 438 

reproductive system of, 501, 502 

trochophore larva of, 501 
Anomostraca, 561 
Anomura, 561, 562 
Anopheles, 768 
Ant bear, 66 1 
Ant eater, spiny, 176 
Antedon, 144 
Antelope, 666 

excretion in, 489 
Numbers refer to paragraphs; Black Face numbers refer to figures. 


44 o 


Antenna, one of the first or second 
pair of jointed appendages, 
"feelers," on the head of an 

Antennae, of moth, 78 

Antennule, one of the first pair of 
jointed appendages on the 
head of a Crustacean. 

Anterior, 311 

Anther, 123 

Antheridium, 264 

Anthocerotaceae, 270 

Anthozoa, 529 

Antibodies, 777 

Antimere, 335 

Antitoxin, 776, 777 

Antlers, 348 

Ant lion, 585 

Ants, 590 

polymorphism in, 740 

Anura, 640 

Anvil, 403 

Apanteles, 236 

Apes, 652, 668 

Aphids, 227, 590 

Apical cell, 710 

Aplanospores, non-motile spores, 247 

Apothecium the concave fruiting sur- 
face of a fungus, 258 

Appendages, of Arthropods, 412 
of Vertebrates, 170, 413, 792 

Apteryges, 650 

Apterygogenea, 577 

Apteryx, 175, 650 

Aquatic plants, 184-191 

Aqueous humor, 382 

Arachnoidea, 566 

Araneida, 569 

Archaeop teryx, 247, 806 

Archegoniates, those cryptogams 
which bear archegonia, viz., 
Bryophyta and Pterido- 
phyta, 266 

Archegonium, 264 

Archenteron, the primitive intestine, 


Arethusa, 848 

Aristotle's lantern, 555 

Armadillo, 421, 652, 661 

Artemia, 687 

Arthostraca, 561, 563 

Arthropoda (Gk. jointed foot), 557 

Arthropods, appendages of, 412 
chemical sense of, 370 
circulatory system of, 471 
digestion in, 458, 459, 467 

Arthropods, eyes of, 378 

exoskeleton of, 417 

glands of, 350 

growth in, 731 

locomotion of, 412 

nervous system of, 443 

segmentation of, 336 

sense organs of, 361 
Artiodactyla, 666 
Ascaris, 687, 695, 758 
Aschelminthes, 538 
Ascidians, 621 

structure of, 621 

symmetry of, 321 
Asclepias, 851 
Ascomycetes, 256 
Ascus (Gk. a bag), 256 
Ash, 64 

Aspidobranchia, 600, 601 
Assimilation the process of trans- 
forming food into the living 
substance, 6, 486 
Association fibres, 439-441, 
Asteroidea, 552 
Asymmetry, 318 
Atrium, 621 

Attraction sphere a rounded mass 
of the cytoplasm which en- 
closes the centrosome, 685 
Auditory organ, 396 

meatus, 401 
Auricle, 401 
Australian fauna, 825 
Autobasidiomycetes, 255 
Autogamy self fertilization, 144 
Aves, 646 

structure of, 646 
Axil, 56 

Axillary bud, 56 
Axis of locomotion, 310 

Babesia, 771 
Bacillus, 227 

aceti, 229 

anthracis, 231 

coli communis, 231 

diphtherise, 231 

pneumonias, 231 

tetani, 231 

tuberculosis, 231 

typhi, ^231 

vulgaris, 230 
Bacteria, 58-61, 180, 217, 227-231, 

673, 778 

as parasites, 772, 773 
nucleus of, 673 

Numbers refer to paragraphs; Black Face numbers refer to figures. 



Bacterium, 227 

Balancers, 587 

Balanoglossus, syn. for Dolichoglossus 

Baleen whale, 665, 796 

feeding of, 844 
Ball and socket joint, 413 
Barnacles, 560, 739 

symmetry of, 321 
Basidiomycetes, 252 
Basidium, 252 
Basilar membrane, 398 
Basket fish, 147, 554 ^ 
Bast, the fibrous portion of the back, 


Bat, 652 
Beak, 347 
Bears, 652, 663 
Beavers, 652, 660 
Bed bugs, 591 
Bees, 590 

Beggiatoa alba, 229 
Berry, 153 

Bilateral symmetry, 315 
Bile pigments, 483 
Bimana, 669 
Biology, i 
Birds (see Aves). 
Birds and reptiles, 792 
Birds, development of, 806 

glands of, 350 

migration of, 843 

reptilian characters of, 649 
Blackbirds, 650 
Black mold, 250 
Blastula, 196, 197, 712 

of lancelet, 712 
Blood, origin of, 721 

-vessels, origin of, 721 
Blue crab, ecdysis of, 76 
Blue flag, pollination of, 253, 254 
Blue-green algae, nucleus of, 673 
Blue molds, 257 
Body cavity, 469 

fluid, 469 

Bojanus, organ of, 607 
Bone, 95, 426 
Bones, origin of, 721 
Bony labyrinth, 399 
Book gills, 565 

lungs, 567, 568, 569 
Bot fly, 761 
Brachiopoda, 549 
Brachyura, 561, 562 
Bract the leaf, usually modified, 
which subtends a floral shoot. 
Brain, 444, 580 

Brain, of annelids, 438 
of crayfish, 100 
of shark, 102 
Branchioganoidea, 632 
Branchiata, 558 
Branchiura, 559 
Brand spores, 253 
Brittle star, 146, 554 . 
Bryinae, 279 
Bryophyllum, 34 

Bryophyta Bryophytes, 176, 264 
Bryozoa, 548 
Budding, 330, 671 
in hydra, 499 
multiplication by, 169 
Buffalo, 666 
Bugs, 763 
Bulbus arteriosus, a muscular chamber 

of the heart, in front of the 

ventricle, 628 
Bull-bat, 650 
Butterflies, 586 

polymorphism in, 743 
Byssus, silky fibres spun from a gland 

in the foot of certain Lamelli- 

branchs, 610 

C symbol for carbon. 

Cactus, 24 

Caddice worm nets, 251, 252 
shelters, 73, 74 

Caecum ccecum cecum, a pouch 
or sac. 

Calyptra, part of the archegonium 
which remains attached to 
the spore capsule, 277 

Calyx, 122, 131 

Cambium, 75 

Camel, 666 

Cameleon, 645 

Campodea, 159 

Canaliculi, of bone, 426 

Canal system of sponge, 521 

Canidae, 663 

Capillary, small blood-vessels having 
walls composed of a single 
layer of thin cells. 

Capitulum, 119 

Caprella, 150 

Capsule, 163 

Carapace, a hard case or shell, 643 

Carbohydrates organic compounds of 
carbon hydrogen and oxygen, 
with the hydrogen and oxy- 
gen in the proportions of 
H 2 0, 94 


to paragraphs; Black Face numbers refer to figures. 



Carbohydrates, absorption of, 472 
Carbon dioxide, formed in body, 
488, 489 

used by plant, 85 
Carbon of plant, source of, 89 
Carinatae, 650 
Carnivora, 652, 662 
Carnivorous : flesh-eating. 

plants, 218 
Carp, 636 
Carpels the leaves modified to form 

the pistil, 28, 125 
Carpogonium, 248 
Carpospores, 248 
Cartilage, 49, 425 

origin of, 721 
Caryopsis, 157 
Cassia, 42, 43 
Cassowary, 650 
Catarrhina, 668 
Catbird, 650 
Caterpillar, 723 
Catfish, 636 
Catkin, 119 
Cats, 663 
Cattle family, 666 
Cave fauna, 838 
Cedar apple, 750 
Cell, 2, 16, 179 

colonies, 704 

division, 683 

masses, 705 

membrane, 672 

regeneration of, 680 

wall, 622 
Cellulose, 96, 619 
Central nervous system, 325 

of annelids, 438 

origin of, 715 
Centrosome, 677, 683, 689 
Centipede, 576 
Cephalization, 311 
Cephalomya, 762 
Cephalopoda Cephalopods, 611, 616 

structure of, 611 
Cercaria, 754 
Cermatia, 576 

Cestoda Cestodes, 537, 755 
Cetacea, 652, 665 
Chaetopoda, 543 
Chaetopterus, tube of, 72 
Characeae, 244 
Chela, a pincer-like claw. 
Chelicerae, the first pair of appendages 
of certain Arthropods, 567 

Chemical sense, 353 

of arthropods, 370 

of crayfish, 369 

of earthworm, 368 

of hydra, 367 

of amoeba, 353 
Chilopoda, 576 
Chimpanzee, 668 
Chinch bugs, 591, 763 
Chiroptera, 652, 659 
Chiton, 341, 593 
Chlorophyceae, 238 
Chlorophyll, 82 
Chloroplasts, 82 

position in cell, 206 
C 6 HioO 6 starch, 87 
CeH^Oe sugar, 90 
Chondroganoidea, 633 
Choroid layer, 379 
Chromatin, 674 

Chromatophore a cell containing 
pigment or, a protoplasmic 
granule containing pigment. 
Chromoplast a protoplasmic gran- 
ule (plastid) containing pig- 
ment, 672 
Chromosome, 683 
Chromosomes, division of, 684, 695 

individuality of, 703 

number of, 687, 695 
Cicada, 396, 591, 763 
Ciconiae, 650 
Cilium(pl. cilia), minute thread-like, 

vibratile appendages. 
Ciliary body, 381 
Ciliata, 514 
Circulation, 327, 468- 
Circulatory system of annelids, 469, 

of arthropods, 471 

of crayfish, no 

of man, 114 

of Nereis, 112, 113 

of vertebrates, 472 
Circumvallate papillae, 371 
Cirratulus, 140 
Cirripedia, 560 
Civet cat, 663 
Clam, 166 
Clamatores, 650 
Classes of animals, 507-669 

of plants, 170-181, 224-301 
Claws, 347 
Cleavage, 702- 

typesof, 711, 718 

Numbers refer to paragraphs; Black Face numbers refer to figures. 



Cleistogamic flowers which do not 
open and which are self 
fertilized, 145 

Climate and vegetation, 223 

Climbing plants, 212 

Cloaca, the chamber into which the 
intestine, ureters, and gono- 
ducts, all open, 556, 637 

Closteridium butyricum, 229 

Club moss Lycopodium. 

Clypeaster, 65, 148 

Cnidaria, 524 

Cnidoblasts, 524 

COz symbol for carbon dioxide, 66 

Coal, 219, 221 

Coccus, 227 

Coccygomorphae, 650 

Cochlea, 398 

Cockroaches, 582 

Cocoon, 586, 590 

Ccelenterates, 518, 

chemical "sense of, 367 
digestion in, 454, 465 
nerve cells of, 435 
sense organs of, 359 

Ccelomata, 532 

Coleoptera, 589 

Collar, of Dolichoglossus, 618 

Coloration, protective, 86 1, 862 

Color of animals, 308, 860 

Color changes, 868 

Columbae, 650 

Columella, 270, 403 

Commensalism, 745 

Composition of plants, 63-65 

Compound eyes, 378, 580 

Conchiolin, 605 

Conchifera, 594 

Condor, 650 

Condylarthra, 666 

Cones of retina, 380 

Coney, 666 

Congo snake, 243 1 

Conidium spores formed in fungi 
by budding, 249 

Coniferae, 296 

Conjugate, 243 

Conjugation, 182, 186, 692- 

Conjugation in protozoa, 498, 699 

Connective tissue, 96, 325 

Connective tissue, origin of, 721 

Conus arteriosus, a chamber of the 
heart lying in front of the 
ventricle and containing sev- 
eral valves, 628 

Copelata, 620 

Numbers refer to paragraphs; Black Face 

Copepoda, 559 

Coral, 75, 135-138, 349, 529 

Cordyceps, 229 

Cork, 101 

Cork cambium 101 

Corm, in 

Cornea, 379 

Corolla, 122, 132 

Corrodentia, 583, 740 

Corydalis, 585 

Corymb, 119 

Cotyledons, 294 

Crabs, 561 

Crab, lung of, 563 

terrestrial, 563 
Cranes, 650 
Cranial nerves, 445 
Crayfish, 561, 563 

chemical sense of, 369 

development of, 504, 505 

digestion in, 467 

green gland of, 491 

integument of, 341 

locomotion of, 412 

reproduction in, 503, 504 

respiration in, 477 

statocyst of, 390 
Cricket, 396, 582 
Crinoids (sea lilies), an order of 

Crocodile, 644 
Crop, 579 

Cross fertilization, 138, 142 
Cross-striped muscle fibre, 414 
Crows, 650 
Crustacea, 558 
Cryptogams, 174-181, 220, 292 

reproduction in, 181 
Ctenidia, comb-like gills, 593, 595 
Ctenobranchia, 600, 601 
Ctenoid (of a fish scale) with a spiny 

edge, 636 
Ctenophora, 531 
Cuckoo, 650 
Cumacea, 561 

Cuscuta (dodder) a genus of para- 
sitic flowering plants. 
Cuticula, of hydroids, 339 

of w6rm, 340 

Cutinized cell walls, 97, 98 
Cuttle fish, 611 
Cyanophyceae, 232 
Cycadinae, 294 
Cycloganoidea, 635 
Cycloid (of a fish scale) with smooth 

rounded edge, 635 
numbers refer to figures. 



Cyclosporeae, 247 

Cyclostomata Cyclostomes, 627 

structure of, 627 
Cyme, 118 

Cymose inflorescence, 25 
Cynips, 764 
Cyprepedium, 850 
Cypselomorphae, 650 
Cysticercus, 755 
Cystoflagellata, 512 
Cytoplasm, 672 

division of, 685 

function of, 680, 682 

Daddy-long-legs, 570 
Damsel flies, 584 
Darning needle, 582 
Dasypeltis, 844 
Day flies, 584 
Decapoda, 561, 616 
Decay, 670 

Deciduous, falling off (especially of 

plants, 197-198 
Deer family, 666 
Degeneration, 588 
Dehisce (of a fruit) to open. 
Dentine, the bony substance of teeth 
lying beneath the enamel, 348 
Dermis, 70 

of mammal, 343 

origin of, 721 
Desmidiaceae, 235 
Development of crayfish, 504, 505 

of insect, 505 

of vertebrate, 506 

progressive, 738 

regressive, 738 

types of, 707 
Devil fish, 167 
Devil ray, 169 

Devil's horse praying mantis, 582 
Diadelphous, 133 

Diastatic (of a ferment) having the 
power of converting starch 
into sugar. 
Diatomae, 233 
Diatomes, growth of, 727 
Dibranchiata, 616 
Dichogamy, 31, 142 
Dicotyledons, 171, 172, 300 

development of, 709 
Difflugia, i 
Differentiation, 323, 725 

of tissues, 22, 96 
Digestion, 326, 452- 

Digestion by bacteria, 453 

in amoeba, 452, 453, 464 

in arthropods, 458, 459, 467 

in ccelenterates, 454, 465 

in crayfish, 467 

in flatworms, 454 

in hydra, 454 

in sponges, 454 

in worms, 455-457, 466 
Digestive ferments, 463 

glands of vertebrates, 461 

tract of crayfish, no 
of grasshopper, 161 
of man, in 

of Nereis, 104-107, 109 
of vertebrates, 460 
Digitigrade (of certain mammals) 

standing on the toes, 663 
Dimorphic two types of form in the 

same species, 143 
Dimorphism, 225, 226 

seasonal, 742 

sexual, 739, 856 
Dinoflagellata, 512 
Dinornis, 650 
Dinornithes, 650 
Dioecious, 128 
Dionaea, 50-52 
Diphycercal, 631, 632 
Diplopoda, 575 
Dipnoi, 631 

Diprotodontia, 652, 656 
Diptera, 587 
Discomycetes, 258 
Dispersal larvae, 831 
Divers, 650 
Division of labor, 725 
Dodder, 230, 751 
Dodo, 650 
Dogs, 652 
Dogwood, 33 
Dolphin, 652, 665 
Dolichoglossus, 618 

gill slits of, 618 

nervous system of, 618 

notochord of, 618 

structure of, 618 
Domesticated animals, 820 

plants, 820 
Dominant characters (in heredity), 


Doodle bug, 585 
Dorsal, 312 
Dove, 650 
Dragon flies, 584 
Dromedary, 666 

Numbers refer to paragraphs; Black Face numbers refer to figures, 



Drosera, 49 
Drupe, 154 
Ducks, 650 
Duck bill, 177, 652 

mole duck bill. 
Dugong, 652, 667 

Eagles, 650 

Ear, 84 

Ear drum, 401-403 

Ear of insects, 397 

Ear of vertebrates, 398 

Ear sacs, 387 

Ear stone, 392 

Earthworm, chemical sense of, 368 

locomotion of, 411 

respiration in, 476 
Earthworms, 305, 543 
Earwigs, 582 

Ecdysis, molting, or shedding, the 
superficial layers of the in- 
tegument, 214, 731 
Echidna, syn. for Tachyglossus, the 

spiny ant eater. 
Echinodermata, 550 
Echinoderm larva, 64 
Echinoidea, 555 

Ecology. The science of the rela- 
tion of organisms to each 
other and to their inanimate 
environment, 182-223 
Ectoderm, 713 

of hydra, 339 
Ectoplasm, 672 
Edentata nomarthra, 652, 66 1 

xenarthra, 652, 661 
Eels, 636 

Effectors, 439-441 
Efts, 639 
Egg, 671 

apparatus, the egg nucleus and 
two other nuclei, synergids, 
which lie together at one end 
of the embryo sac, 299 

maturation of, 696 

membrane, 702 

of bird, 647 

pronucleus, 698, 699 

size of, 697 

yolk of, 711 
Eimeria, 127 
Elaters, 267, 287 
Electrical organs, 874 
Electric eel, 636, 874 

light bug, 591 
Elephants, 666 

Elytra, 589 
Embryo, 43, 44 

human, 249 

of dicotyledon, 189, 709 

of fern, 188, 709 

of hydra, 123 

sac megaspore, 293 
Emeu, 650 

Enamel, of scales, 348 
Endolymph, 398 
Endoskeleton, the internal skeleton, 

420, 422- 
Endospores, 228 
Endosperm, 53 

gametophyte, 299 
Endothelium a thin layer of cells 

lining a cavity, 379 
Energy of animals, source of, 484 

relations of the animal, 446 
Entamceba, 765 

coli, 510 

histolytica, 510 
Enteropneusta, 618 
Entoderm, 713 
Entomophilous flowers which are 

pollinated by insects 
Emydosauria, 644 
Environment, 332 
Ephemeroidea, 584 
Ephyra, 528 

Epidermal hairs, of plants, 21 
Epidermis, 20, 70, 101 

of leaf, 27, 80 

of mammal, 343 

of stem, 73 

of worm, 340 

origin of, 715 
Epigynous, 134 

Epiphysis, the smaller bone formed 
on the end of a long bone, 
with which it ultimately 

Epiphytes, 213 
Epithelium, a layer of cells covering 

an organ or lining a cavity. 
Equilibration, 366 

in vertebrates, 392-395 
Equisetinae, 287 
Equisetum, 287 
Eustachian tube, 403 
Euthyneura, 600, 602 
Evolution, 781- 
Excretion, 329, 489- 

by liver, 493 

in amoeba, 489 

in hydra, 489 

Numbers refer to paragraphs; Black Face numbers refer to figures. 



Excretory organs of Nereis, 119, 120 
Exoasci, 26 1 'I 

Exogen, a plant whose stem increases 
in thickness by growth in the 
region between the bark and 
the wood. 
Exoskeleton, an external skeleton. 

of arthropods, 417 

of molluscs, 418 

of vertebrates, 419 
Eye, control of light intensity in, 386 

focusing of, 385 
Eyes, 77, 79, 80 

of arthropods, 378 

of cave fishes, 796 

of insects, 384 

of vertebrates, 379 

of worm, 376, 377 
Eyespot, in protozoa, 374 
Eyespots, 526, 528 

F, FI, 2 first filial, second filial 

and third filial generations. 
Falcon, 650 

Fascia, sheets or layers of connective 
tissue covering organs or 
forming attachment for mus- 
cles, 721 

Fasciola a liver fluke. 
Fats, 473 
Fauna of Africa, 826 

of Australia, 825 

of caves, 838 

of fresh waters, 831-835 

of oceanic islands, 827 

of South America, 826 
Feathers, 347, 646 
Feigning, 864 
Felidae, 663 
Female, 694 

Ferment, a substance which causes a 
chemical change in other sub- 
stances without undergoing a 
permanent change itself. 
Fermentation, 230, 453, 670 
Fern, development of, 708 

sperms of, 700 
Ferns, 281 
Ferret, 663 
Fertilization, 135, 187, 694, 698 

in hydra, 500 

stimulus, 700 
Filament, 123 
Filices, 285 
Filicinae, 281 
Finches, 650 

Fire-fly, 220, 872 

Fire-flies, rudimentary wings of, 797 

Fishes, glands of, 350 

respiration in, 479 

skeleton of, 424 

sense of taste of, 370 
Fission reproduction by division, 671 
Fissipedia, 663 

Flagellata Flagellates, 512, 766 
Flagellum a whip-like protoplasmic 

Flame cell, 534 
Flamingo, 650 
Flat fishes, 322, 636 

worms, digestion in, 454 
Flea, 237, 762 
Flies, 587 

Flight, adaptation for, 646 
Floral structures, 27 
Floridese, 248 
Flounder, 66 

symmetry of, 322 ' 
Flower, function of, 135 

homology of, 115-120 
Fly catcher, 650 
Fcetal membranes, 173, 174, 641 

of mammals, 651 
Foliate papillae, 371 
Follicle, a minute cavity, sac, or tube, 


Food of animals, 447-451 
Foot, of fern, 282, 708 

of gastropods, 595 

of molluscs, 592 

of mosses, 274 

of rotifers, 539 
Form of animals, 308 

of organisms, 7 
Four-o'clock, 242, 782 
Fowl, 650 

Free nerve terminations, 362 
Fresh water fauna, 831-835 
Frigate bird, 650 
Frog, metamorphosis of, 211 

respiration in, 476 
Frogs, 637, 640 
Fruit, 149, 151 
Fruits, aggregate, 164 

kinds of, 152-165 

function of, 168 . 

multiple, 165 

simple, 152-163 
Fucus, fertilization of, 693 
Fulgur, shell of, 213 
Function of the senses, 405 
Fungi, 177, 179-180 

Numbers refer to paragraphs ; Black Face numbers refer to figures. 



Fungiform papillae, 371 
Fur seal, 739 

Gaillardia, 241 
Galeorchis, 849 
Gallinacei, 650 
Galls, 764 
Gall wasp, 238, 590 
Gametangium (of plants) the organ in 
which gametes are produced. 
Gametes, 694 
Gametophyte. 265, 292 
Gamopetalous, 132 
Gamosepalous, 131 
Ganoin, 632 
Garpike, 419 
Garpikes , 634 
Gastric fluid, 641 

glands, 720 

mill, 459 
Gastrophilus, 761 
Gastropoda, 595 

structure of, 595- 
Gastro-vascular cavity, 454 
Gastrula, 713 

cavity, 713 

mouth, 713 
Gastrulation, 198, 199 
Gavial, 644 
Geese, 650 

Geographical distribution, 823 
Geotropism, 49, 50 
Germ, 9 

Germinal tissue, 724 
Germination, 46-50 
Germs, 670 
Gibbons, 668 
Gills, 593, 606 

of crayfish, 477 

of fishes, 479 
Gill slits of Dolichoglossus, 618 

of vertebrates, 813, 814 
Ginkgo, 295 
Ginkgoinae, 295 
Giraffe, 666 
Gizzard, 579 
Glands, 71, 349~3S2 

of arthropods, 350 

of birds, 350 

of fishes and amphibia, 350 

of hydra, 349 

of mammals, 351 

of reptiles, 350 
Glandular activity, 487 
Glossinia, 766 
Glowworm, 739 

Glucose a kind of sugar CeHiaOe 

(grape sugar). 
Glycogen, 472 
Gnats, 587 
Gnetinae, 297 
Goats, 666 

Golden thread dodder 
Gonad the organ in which the germ 
cells (gametes) are developed. 
Gonads, of hydra, 500 

of insects, 580 

origin of, 721 
Gordius, 756 
Gorilla, 668 
Grallae, 650 
Grasshopper, 396, 582 
Gray matter of brain and spinal cord, 


Grebes, 650 

Green gland, of crayfish, 491 
Growth, 6, 486 
Grub, 587 
Grubworm, 589 
Guard cell one of the epidermal cells 

which bound a stoma, 86 
Guinea-pig, 660 
Gulls, 650 

Gustatory sense sense of taste. 
Gyncecium, 125, 128 
Gymnophiona, 638 
Gymnospermae Gymnosperms, 173- 

Gymnotus, 874 

H symbol for hydrogen. 
Haemocyanin, 483 
Haemoglobin, 483 
Hair, 347, 651 

follicle, 70 
Hairs, of arthropods, 346 

of plants, 100 
Halteres, 587 
Hammer, 403 
Harvestmen, 570 

Harvey, William English anatomist 
who discovered the circula- 
tion of the blood (1578-1657), 

Haversian canals, 426 
Hawks, 650 
Hearing, 366 

and equilibration, 387- 
Heart, 471, 472 

origin of, 721- 
Hedgehog, 652 
Heliotropism, 50 

Numbers refer to paragraphs; Black Face numbers refer to figures. 



Heliozoa, 510 
Hellgrammite, 585 
Hemibasidialis, 252 
Hemiptera, 763 

wings of, 797 
Hepaticae, 267 
Hepatic, pertaining to the liver. 

caeca, 553 

portal vein, the vein which leads 
from the intestine to the liver, 
Herbivorous feeding on vegetable 

Heredity, 782 

Mendel's laws of, 782 

physical basis of, 785 
Hermaphrodyte having the organs of 
both sexes in one individual, 

Hermit crab, 562 
Heron, 650 
Herring, 636 
Heterocercal, 633 
Heteronymous, 336 
Heteropoda, 600, 60 1 
Heterotricha, 515 
Hibernia (moth), 245, 246, 739 

moth, rudimentary wings of, 797 
Hibernation, 842 

"Higher" and "lower" animals, 332 
Hinge joint, 413 

ligament, 605 
Hippopotamus, 666 
Hirudinea, 546 
H 2 O symbol for water. 
Holocephali, 630 
Holophytic, 744 
Holothuria, symmetry of, 320 
Holothuroidea, 556 
Holotricha, 515 
Holozoic, 744 
Hominidae, 669 
Homo, 669 
Homonymous, 336 
Hoofs, 347 
Horns, 347 
Horse, 666 

evolution of, 248, 807 

-hair snake, 756 

-shoe crab, 565 
Host an organism that harbors a 

Hosts, of bacteria, 773 

alternation of, 769 
Humming birds, 650 
Hyaenidae, 663 

Hybrid, 147, 781 
Hydra, 69, 304 

budding in, 499 

chemical sense of, 367 

digestion in, 454 

excretion in, 489 

glands of, 349 

gonads of, 500 

integument of, 339 

muscle fibres of, 408 

reproduction in, 499 

response in, 408 

sense organs of, 358 

sensitiveness to light, 375 

sexual reproduction in, 500 

supporting lamella, 415 
Hydractinia, 222 
Hydroid, IM. 222 
Hydrophytes, 103- 192 
Hydropterides, 286 
Hydrotropism, 49 
Hydrozoa, 525 
Hyena, 663 
Hymenoptera, 590 

polymorphism in, 740 
Hypha one of the filaments which 
make up the mycelium of a 
Hypocotyl, 52 
Hypoderma, 761 

Hypogynous (of stamens, etc.) at- 
tached below the ovary. 
Hypotricha, 515 
Hyracoidea, 666 
Hyrax, 666 

Ibis, 650 
Ichneumon, 663 

fly, 235, 590, 762 

Imago, 586 
Immunity, 775 

acquired, 777, 780 

natural, 776, 780 

active, 778, 780 

passive, 779, 780 
Impennes, 650 
Incus anvil. 

Indehiscent (of fruits) not opening. 
Indian pipe, 751 
Indirect development, 722 
Individual, 725 
Indusium, the membrane covering or 

enclosing a sorus, 285 
Inferior (of ovary) below the calyx, 

Inflorescence, 117-120 

Numbers refer to paragraphs: Black Face numbers refer to figures. 



Ink gland, 611 
Insecta insects, 578 
Insects, development in, 505 

growth in, 732 

integument of, 341 

of oceanic islands, 836 

respiration in, 478 

sense of smell of, 369 
of taste of, 369 

voice of, 396 

wings of, 412 
Insectivora, 652, 658 
Insertion, of muscle, 429 
Inspiration, 481 
Instinct, 875 
Integument, 70, 324, 338 

of arthropods, 342 

of insects, 341 

of mammals, 343 

of vertebrates, 733 

special structures of, 346, 347 
Intelligence, 880 
Internodes the portion of the stem 

between two nodes. 
Intestinal glands, 720 
Invertebrates, eyes of, 379 
Invertin, 462 
Involucre, 120 
Iris, 379 
Isoetaceae, 291 
Isoetes, 291 
Isopoda, 563 

Jaws, 346 
Jelly fishes, 528 

nervous system of, 437 
Jungermanniaceae, 271 

Kangaroo, 652 
Karyokinesis mitosis, 686 
Katydid, 396, 582 
Kidneys, 329, 492 
origin of, 721 
Kidney tubule, 492 
King bird, 650 
Kiwi. See also Apteryx, 650 

Labium, 578 

Labyrinth, 392, 393, 398, 399 

Lace wing flies, 585 

Lacertilia, 645 

Lacteals, the lymphatic vessels of the 


Lacteal capillaries, 472 
Lactuca, 38, 39 
Lacunae, 426 

Lady slipper, 850 
Lamellae of bone, 426 
Lamellibranchiata, 604 

structure of, 604 
Lamellirostres, 650 
Lancelet, 625 

egg, cleavage of, 712 

notochord of , 422 

structure of, 625 
Lantern fly, 163 
Lateral line, 404 
Latitude and plants, 204 
Lari, 650 
Larks, 650 
Larvae, dispersal, 722 
Larva, the young of an animal before 
metamorphosis, when devel- 
opment is indirect. 

of hydra, 500 

of the lancelet, 200, 210 

of tunicates, 621 

trophic, 723 

Law of Biogenesis, 809, 812 
Leaves, form of, 23 

storage, 114 

venation of, 26 

Leaf, structure of, 14, 16, 24, 80 
Leech, locomotion of, 411 
Legume, 162 
Lemurs, 668 
Lens of eye, 381 
Leopards, 663 
Leptocardia, 625 
Lepidoptera, 586 
Leptothrix, 227. 229 
Leptostraca, 561 
Lice, 583 

Lichens, 48, 216, 263, 745 
Life habits, 744 
Ligaments, 428 

origin of, 721 

Light and plants, 205-208 
Light sense in protozoa, 374 
Lignified cell- walls, 97 
Lilac mildew, 748 
Lime, secretion of, 349 
Limulus, 151, 565 
Linguatulida, 572 
Linin, 675, 690 
Lions, 663 
Liver, 472, 493, 720 

excretion by, 493 
Liver fluke, 231, 232, 754 
Liverworts, 264, 267 
Lizards, 645 

rudimentary appendages of, 795 

Numbers refer to paragraphs; Black Face numbers refer to figures. 



Llama, 666 
Lobster, 561 
Locomotion, 308, 309 

in annelids, 410, 411 

in arthropods, 412 

in vertebrates, 413 
Lophophore, 548 
Love vine dodder. 
Luminescence, production of light, 869, 


Luna moth, 244 
Lungs, 118 

of crab, 563 

of dipnoi, 631 

of snail, 602 

of vertebrates, 480 
Lycoperdon, 46 
Lycopodiaceae, 289 
Lycopodinae, 288 
Lycopodium, 289 
Lymph, 474 

glands, origin of, 721 

origin of, 721 

spaces, 474 

vessels, 474 
Lysin, 777 

Mackerel, 636 

Macrocystis pyrifera, 245 

Macrogamete egg, 694 

Macronucleus meganucleus, 674, 699 

Macrospore megaspore. 

Macrura, 561, 562 

Maggot, 587, 670 

Malacostraca, 561, 563 

Malapterurus, 874 

Malarial fever, 768 

Male, 694 

Males, dwarf, 739 

Malleus hammer. 

Malpighian tubules, 579 

Maltose a kind of sugar Ci 2 H 22 On. 

Mammalia mammals, 651 

Mammals of Australia, 825 
integument of, 343, 344 
of glands of, 351 

Mammary glands, 651 

Man, 652 

Manatee, 652, 667 

Mandible, jaw, the lower jaw of ver- 
tebrates or one of the first 
pair of appendages of the 
mouth in arthropods. 

Mandrels, 668 

Mangrove seedling, 259 
tree, 854 

Mantle of molluscs, 592 

Marchantiaceae, 269 

Marsh plants, 192 

Marratiacese, 284 

Marsupialia, 652, 654, 655 

Mastigophora, 511 

Maturation, 695 

of ovum, 184, 185, 785 
of sperm, 786 

Maxilla, one of the second or third 
pair of appendages of the 
mouth of arthropods, 578, 

Mechanics of growth, 212-218, 727 

Mechanism of response, 441 

Medullary groove, 206, 207, 715 
plate, 204, 205, 714, 7i5 
ray, 62 
tube, 209, 715 

Medusa, 525, 528 

Medusae, light sense organs of, 375 

Meganucleus macronucleus, the 
larger of the two nuclei of cer- 
tain protozoa. 

Megaspore, 286 

Membrane bone, 427 

Membrana tectoria, 398 

Membraneous labyrinth. See laby- 

Mendel's laws of heredity, 782 

Meristematic, actively growing. 

Mesenchyma, mesodermal tissue which 
arises as separate cells, not 
as a continuous layer. 

Mesenteries, origin of, 721 

Mesocarpaceae, 237 

Mesoderm, 408, 517, 716, 717, 719 

Mesodermic somites, 204-210, 717 

Mesoglea, a gelatinous layer between 
the ectoderm and entoderm 
of ccelenterates. 

Mesophyll, 27, 8 1 

Mesophytes, 183, 195 

Meso thorax, 578 

Metabolism, 484 

nuclear control of, 682 

Metameres, 334 

Metamorphism, 581 

Metamorphosis a marked change of 
form occurring duringdevelop- 
ment, 320, 506, 637, 723 
in insects, 505 
in tunicates, 621 

Metathorax, 578 

Metazoa, 517 

Mice, 652, 660 

Numbers refer to paragraphs; Black Face numbers refer to figures. 


45 1 

Mice, inheritance of color in, 783 
Microgamete sperm, 694 
Micronucleus, the smaller of the two 
nuclei of certain protozoa, 
674, 699 

Micropyle, a small hole, 701 
Microsome, a small body, 672 
Microspore the smaller spore, when 
there are two kinds. It gives 
rise to a male gametophyte, 

Microspora, growth of, 728 
Middle ear, 87, 403 
Migration of birds, 843 
Mildew, 257, 748 

Milkweed, pollination of, 255, 256, 851 
Mimicry, 865-867 
Mink, 663 

Mirabilis Jalapa. See four o'clock. 
Mistletoe, 663, 751 
Mitqs, 571 

Mitosis, 180, 181, 479, 686 
Moa, 650 
Mocking bird, 650 
Modified roots, 103-106 

stems and branches, 107-113 
Moles, 652 
Mollusca, 59_2___ 
Mollusc, exoskeleton of, 418 

foot of, 592 

mantle of, 592 

segmentation of, 333 

shell of, 349, 592 

structure of, 592 
Molluscoidea, 547 
Monotfemata, 652, 653 
Monkeys, 652, 668 
Monocotyledons, 171, 172, 300 

growth of, 729 
Monodelphia, 652, 657 
Monodelphous, 133 
Monoecious, 128 
Morchella, 258 
Mosquitoes, 587, 768 
Mosses, 264, 273 

sperms of, 700 
Mother of pearl, 605 
Moths, 586 

Mougeota, conjugation in, 693 
Mouth parts of cockroach, 160 

of insects, 459 
Mucous epithelium, 720 
Mud puppies, 639 
Mullet, 636 
Multiple fruit, 165 
Multicellular body, 706 

Multipolar nerve cells, 435 
Muscle, 325, 429 

fibres, 89-92, 408, 409 
Muscles of annelids, 408-411 

of vertebrates, 414 

origin of, 721 

stimulus, 431 
Muscular activity, 488 

contraction, 430 
Mushrooms, 255 

Mycelium the mass of threads which 
constitute the vegetative body 
of a fungus. 
Mycorhiza, 216 
Mycetozoa, 225 
Mygale, 153 
Myxomycetes, 225 
Myonemes, 88, 407 
Myriapoda, 574 
Myrrh, 261 
Myxamceba, 225 

N symbol for nitrogen. 
Nails, 347 
Narwhal, 665 
Natural selection, 819 
Nautilus, 1 68, 6n 
Nematoda, 540 
Nemertini, 541 
Nephridia, 329, 593 
Nephridia of worms, 490 
Nereis, 67, 305, 543 

locomotion of, 411 

respiration in, 476 

sense organs of, 360 
Nerve cells, 97, 100, 101 
of coelenterates, 435 
types of, 435 ^ 

-muscle mechanism, 325 
Nervous system, 434 

of annelids, 438 

of arthropods, 443 

of dolichoglossus, 618 

of jelly fish, 437 

of nereis, 98 

of tunicates, 619 

of vertebrates, 444 
Nettling cells, 524 
Neuroptera, 585 
Newts, 637, 639 
Nictitating membrane, 648 
Nitrate bacteria, 229 
Nitrite bacteria, 229 
Nitrogen bacteria, 217 

waste, 488, 490 
Noctiluca, 512 

Numbers refer to paragraphs; Black Face numbers refer to figures. 

45 2 


Node the point on a stem at which 

the leaves are borne. 
Non-ruminants, 666 
Notochord, 93, 206^210,422,42^, 716 

of dolichoglossus, 618 

of tunicates, 619 
Nuclear membrane, 674, 685 

sap, 676 

spindle, 683 
Nucleolus, 674 
Nucleoli, 688 
Nucleoplasm, 679 
Nucleus, 1 6, 673 

function of, 680-682 

resting, 691 

structure of, 674 
Nut, 158 

Oak galls, 238 

Oceanic islands, fauna of, 827 
insects of, 836 

Ocelli, 580 

Octopus, 611, 616. (See also devil- 

Odonata, 584, 585 

(Edogonium, 739 
growth of, 728 

(Esophagus of crayfish, 458 

Oil, 350, 351, 672 

Oils, vegetable, 94 

Olfactory sense. (See sense of smell.) 

Oligochaeta, 545 

Oligotricha, 515 

Ommatidium, 79, 378, 384 

Omnivorous using all kinds of foods. 

Ontogeny, the history of the develop- 
ment of the individual. 

Ontogenetic series, 809 

Oogonium (in the lower cryptogams) 
a cell within which one or 
more ova are formed. 

Oomycetes, 250 

Operculum, 279, 632, 633 

Ophidia, 645 

Ophioglossaceae, 283 

Ophiuroidea, 554 

Opilionidea, 570 

Opisthobranchia, 600, 602 

Opossum, 652, 654 

Optic nerve, 380 

Opuntia, 24 

Orang-utan, 668 

Orchids, 848 

Organ of Bojanus, 607 
of Corti, 85, 86, 398 

Organism, 4 

Organization of the body, 331 
Organs of response, 406- 

of sight, 374- 

of special sense, 366- 
Origin of species, 789 
Ornithorhynchus. (See duck-bill.) 
Orthoptera, 582 
Oscines, 650 
Osmose, diffusion of a solution through 

a membrane, 69, 71 
Osphradium, an olfactory organ found 

in many molluscs, 597 
Ossification of vertebra, 217 
Ostium, an opening, mouth, 471, 579 
Ostracoda, 559 
Ostrich, 650 
Otters, 664 
Ovary, 29, 125 

of hydra, 121, 500 
Ovules, 29, 126 
Ovum of hydra, 122, 500 

of nereis, 125 
Owls, 650 
Oxidation, 475 
Oxygen, 67 

given off by plant, 87 
Oxygenation of blood, 482 

Palps, a jointed sensory organ attached 

to the mouth appendages of 

arthropods, 580 
Pain, 364 
Palisade cells, 81 
Palaeostraca, 564 
Palisade tissue, 205 
Paludina, development of, 805 
Pancreas, 462, 720 
Panther, 663 
Papilio merope, 267 
Papilionaceous blossom, 846 
Paramoecium, TsS- 

conjugation in, 186, 699 
Parasites, 215 
Parasitism, 747 
Parrots, 650 
Parthenogenesis, development of an 

egg without fertilization, 581 
Parenchyma, 72 
Passeres, 650 
Pasteur (Louis), 1822-95, French 

bacteriologist, 670 
Pearly nautilus, 611, 616 
Pecora, 666 
Pedicl, 116 
Pedipalpi, 568 

Numbers refer to paragraphs; Black Face numbers refer to figures. 



Peduncle, 116 

Pelican, 650 

Pellicle, 338 

Pelmatozoa, 551 

Penguin, 650 

Pentadactyl appendages, 637 

Pepsin, 461, 463 

Peptones soluble proteid compounds, 

absorption of, 472 

Perch, 636 

Perennial, living on from year to year. 

Perianth, 122 

Pericardial cavity, 471 

Pericardium, origin of, 721 

Pericarp the wall of the ovary when 
the fruit is mature. 

Peridermium, 227, 228 

Perigynous, 134 

Perilymph, 399 

Peripatus, 157 

Perisarc, 339 

Perisperm, 53 

Perisporeaceae, 257 

Perissodactyla, 666 

Peris tome, 247 

Periostracum, 605 

Perithecium (of fungi) an urn-shaped 
fruiting receptacle, 257 

Peritoneum, origin of, 721 

Peritricha, 515 

Petiole, 24, 25 

Petals the parts of the corolla, 132 

Phaeophyceae, 245 

Phaeosporeae, 246 

Phagocytosis, the destruction of bac- 
teria by the white blood cor- 
puscles phagocytes, 776 

Phanerogams, flowering plants, 292 

Pharynx, of tunicates, 619 

Phascaceae, 278 

Pheasant, 650 

Phenacodus, 666, 807 

Phloem, the part of the vascular 
bundle containing the sieve 

Phosphorescence luminescence. 

Phosphorus, 679 

Photogenic producing light. 

Photosynthesis the process by which 
organic substances, such as 
starch, are formed by the 
agency of chlorophyll in sun- 
light, 87 

Phycocyanin, 232 

Phycoerythrin, 248 

Phycophaein a brown pigment con- 
tained in the brown sea- 
weeds, 245 
Phycomycetes, 249 

Phyllotaxy leaf arrangement, -30-33 
Phyllopoda, 559 
Phylloxera, 763 

Phylogey, the history of the develop- 
ment of the race. 
Phylogenetic series, 802 
Physalia, 134 
Physiographic relations of plants, 219- 


Pici, 650 
Pigeon, 650 

Pigment layer of eye, 379 
Pike, 636 
Pill bug, 561 
Pine, 35 
Pineal eye, 796 
Pinnipedia, 664 
Pisces, 628 

structure of, 628 
Pistil, 29, 125 

Pistil megasporophyll, 298 
Pith, 6 1 
Placenta, 126 
Plant bugs, 591 
Plants, color of, 18-22 
Plant hairs, 100 
Plantigrade standing on the whole 

sole of the foot, 663 
Plant lice, 591, 763 
Plant series, 793 
Planula, 528 

Plasmodium the amceboid stage of 
the body of a slime mold, 
252, 240, 768 
Plastron, 643 
Platyhelminthes, 534 
Platyrhina, 668 

Pleura, the membrane covering the 
lungs and lining the thoracic 

origin of, 721 
Pleurobrachia, 531 
Plumule, 55 
Pocket-gopher, 660 
Poison gland, 567, 569 
Polar body, 696 
Pole cat, 663 
Pollen, 123 

grain microspore, 293 

protection of, 137 

selection of, 147 

tube nucleus, 294 

Numbers refer to paragraphs; Black Face numbers refer-to figures. 



Pollination, 135-147, 845 

by insects, 140, 143 

by wind, 139 
Polychaeta, 544 
Polygala, 30, 31 
Polymorphism, 221-223, 527, 458, 581, 


in bryozoa, 740 
in hydrozoa, 740 
in plants, 743 
Polyps, 525 
Polypodium, 40, 41 
Polyprotodontia, 652, 655 
Polyzoa, 548 
Porifera, 519- 
Porpoises, 652 
Posterior, 311 
Prawns, 561 
Praying mantis, 582 
Primates, 652, 668 
Proboscidia, 666 
Proboscis, 586 
Procyonidae, 663 

Proglottis, segment of a tapeworm, 537 
Pronuba, 852, 853, 875 
Prosimiae, 668 
Protective coloration, 861, 862 

in grasshoppers, 262, 263 
resemblance, 264, 863 
Proteolytic having the property by 

which proteids are changed 

into peptones, 461 
ferments, 463 
Prothallium, the thalloid gametophyte 

of certain pteridophytes, 280 
Pro thorax, 578 
Protobasidiomycetes, 254 
Protoascales, 240 
Protonephridia, 534, 541 
Protonema, the green thread-like part 

of the gametophyte of mosses, 

Protoplasm, 13 

chemical properties of, 15, 68, 672, 

contractility of, 680, 68 1 

irritability of, 680 

structure of, 178, 672 
Protozoa, 507- 

as parasites, 765 

conjugation in, 498 

sensitiveness to light, 355 
Protracheata, 573 
Pseudopodium 276 
Psittaci, 650 
Pteridophyta-Pteriodophytes, 175, 280 

Ptyalin, 461 

Puff balls, 255 

Pulmonata, 600, 602 

Pupa, 586, 723 

Pupil, 379 

Pygopoda, 650 

Pyrenomycetes, 259 

Python, rudimentary appendages, 795 

Quail, 650 

Queen conch, shell of, 218 

Rabbits, 652, 660 

Raceme, 119 

Racemose inflorescence, 26 

Raccoon, 663 

Radial symmetry, 316 

Rachis, 29 

Radiolaria, 510 

Radula, 593, 596 

Rails, 650 

Ratitae, 650 

Rats, 652, 660 

Ravens, 650 

Receptacle, 122, 130 

Receptors, 439, 441 

Recessive characters, in heredity, 783 

Red blood corpuscles, 483 

Redi, Francesco, 1626-98, Italian 

naturalist, 670 
Redii, 754 

Reduction division, 695, 696, 698 
Regeneration, 726 
Regions of growth, 102 
Reproduction, 330, 495- 

in amoeba, 495, 496 

in annelids, 501, 502 

in crayfish, 503, 504 

in hydra, 499 

in sponge, 523 

in vertebrates, 506 
Reptilia reptiles, 691 
Reptiles, glands of, 350 

structure of, 641 
Reserve food, 672 
Resemblance, protective, 863 
Respiration, 308, 327, 475- 

in amoeba, 476 

in crayfish, 477 

in earthworm, 476 

in fishes, 479 

in frog, 476 

in insects, 478 

in nereis, 476 

in plants, 91 

in sea anemone, 476 

Numbers refer to paragraphs; Black Face numbers refer to figures. 



Respiratory organs of crayfish, 116, 117 
of nereis, 115 

tree, 556 
Response, in amoeba, 407 

in annelids, 409-411 

in hydra, 408 
Retina, 379 

Retinal elements, 81, 380 
Rhabdom, 384 
Rhea, 650 
Rhinoceros, 666 
Rhizoids. 267 
Rhizopoda, 509 
Rhodophyceae, 248 
Rhomboganoidea, 634 
Rhythmical changes in plants 197, 

199 200 

Rhynchocephalia, 642 
Rhynchota, 585, 591 
Ricciaceae, 268 
Rice birds, 650 
Rocks, decay of, 222 
Rodentia, 652, 660 
Rods, of retina, 380 
Root, 7 

cap, 102 

hairs, 8, 68-69 

tip, 23 
Roots, 37, 39 

aerial, 105 

of epiphytes, 106 

prop, 104 

storage, 103 

structure and function of, 68- 71 
Rootstock, in 
Rosette habit, 201 
Rotatoria, 539 
Rotifers, foot of, 539 
Round-mouthed eel, 423, 627, 752 
Round worms thread worms, 540 
Rudimentary organs, 794 
Ruminants, 666 
Runner, 109 
Rusts, 254, 749 

S symbol for sulfur. 
Saccharomycetes, 262 
Sacculina, 560, 739, 760 
Sacculus, 393, 394 
Salamanders, 637, 639 
Salamandra, 171 
Salicornia, 37 
Salivary glands, 459, 720 

of insects, 579 

of man, 461 

of nereis, 455 

Salmon, 636 

Salvia, 461, 847 

Samara, 159 

Sand flea, 561 

Saprophytes, 214 

Saprozoic nourished like fungi, i.e., on 

soluble organic matter. 
Sarracenia, 53-55 
Scale insects, 591, 763 
Scale leaves, 114 
Scales, 70 

bony, 348 

of vertebrates, 347 

of worms, 346 

Scaly ant eater, 421, 652, 66 1 
Schizocarp, 160 
Schizophyceae, 232 
Schizophyta, 226 
Schizopoda, 561 
Sclera, 379 
Sclerenchyma hard tissue, cells with- 

thick walls. 
Scolecida, 533 
Scolex, 537, 755 
Scorpion, 152 
Scorpionidea, 567 
Scyphozoa, 528 
Sea anemone, 529 

chemicatfsense organs of, 367 
respiration in, 476 

cow, 667 

cucumber, 556 

horse, 636 

lilies, 551 

squirt, 621 

turtles, 643 

urchins, 555 

weeds, 208 
Seal, 652, 661 
Seed, the, 148, 151, 292 

distribution, 166-168 
Seedling, 51 
Seeds, 42-45 
Segmentation, of body, 333, 337 

of egg cleavage, 190-195, 702 
Selachii, 629 
Selaginella, 290 
Selaginellaceae, 290 
Self fertilization, 144 
Semicircular canals, 393, 394 
Sense of position, 365 

of smell, 366 
of insects, 369 
of man, 373 
of vertebrates, 370 

of taste, 366 

Numbers refer to paragraphs; Black Face numbers refer to figures. 

45 6 


Sense of taste of fishes, 370 
of insects, 369 
of man, 372 
of vertebrates, 370 
of touch, 364 

of amoeba, 354 
of weight, 365 
organs, 77, 325, 353- 
of amoeba, 353~357 
of arthropods, 361 
of ccelenterates, 359 
of hydra, 358 
of Nereis, 360 
of vertebrates, 362 
special, 311 
Senses of animals, 404 
Sensibility, 309 
Sensory corpuscles, 362 
Sepal one of the parts of the calyx, 131 
Serpent star, 554 
Sessile not raised on a stalk. 
Seta, 279, 346 
Sex, development of, 182 
Sexual dimorphism, 219, 220 
reproduction in hydra, 500 
selection, 857 
Sharks, skeleton of, 423 
Sheep, 666 

Shell of animals, growth of, 730 
of gastropods, 595 
of molluscs, 418, 592 
of turtles, 420 
Shrew, 652 
Shrimp, 149, 561 
Sieve tubes, n, 74 
Sight, 366 
Simise, 668 
Siphon, 605 
Siphonales, 243 
Siphonaptera, 588 
Siphonocladiales, 242 
Siphonophore, 221 
Sirenia, 652, 667 
Sirenidae, 639 
Sitaris, 876 
Size of organisms, limitation of, 8 

and differentiation, 323 
Skeletal muscles, 414 
Skeleton, 325 

and connective tissue, 415 
cartilaginous, 423, 424 
of sponge, 520 

of vertebrates, growth of, 734, 735 
Skull of human embryo, 216 

of vertebrates, 792 
Slaves of ants, 590 

Sleeping sickness, 766 
Slime mold, 57 

molds myxomycetes. 
Sloth, 652 
Slug, 165 
Slugs, 602 
Smell, 366 

Smooth muscle fibre, 414 
Smut, 253, 259 
Snail, 164, 165 
Snails, 595 

fresh water, 602 
Snakes, 645 
Snapping turtle, 643 
Soil and plants, 209-210 

bacteria, 217 
Soldier (of Aphids), 583 
Solenoconchae, 603 
Sole, symmetry of, 322 
Somatic tissue, 724 
Sorus, a cluster of sporangia, 284, 285 
South American fauna, 826 
Sparrows, 650 
Species, 781 

number of, 788 

origin of, 789- 
Spermatia, 248 
Spermatogenesis, 183, 185 
Spermatophyta spermatophytes, 

170, 292 
Sperm cells, formation of, 695 

motility of, 697 

nucleus, 698, 699 
of hydra, 500 

nuclei of pollen, 299 
Sphagnacese, 276 
Sphex, 876 
Spicules, 522 
Spider, 569 

web, 844 

Spiders, polymorphism in, 743 
Spike, 119 
Spinal cord, 103, 444 

ganglia, 363 

nerves, 444, 445 
Spindle fibres, 690 
Spiny ant-eater, 652 
Spiral valve, 627, 629, 631, 632 
Spirillum, 227 
Spirobolus, 158 
Spirochaete, 227 
Spirogyra, conjugation of, 693 
Spirostomum, myonemes of, 88 
Spleen, origin of, 721 
Sponge, 129-132 

structure of, 520 

Numbers refer to paragraphs; Black Face numbers refer to figures. 



Sponges, 519- 

canal system of, 521 

digestion in, 454 

reproduction in, 523 
Spongilla, 519 

Spontaneous generation, 670 
Spore, 181, 671 
Sporidia, 749 

Sporophyll, a leaf bearing spores, 282 
Sporophyte, the generation which pro- 
duces spores asexually, but is 
produced sexually, 265 
Sporozoa, 513, 768, 774 
Spring lizards, 639 

tails, 577 
Spurs, 347 
Squamata, 645 
Squash bug, 591 
Squid, 611, 616 
Squirrels, 652, 660 
Stapes stirrup, 399 
Staphylococcus, 231 
Starch, 18, 87, 672 

changed to sugar, 90 
Starfish, 141-143, MS, 55^553 
Statocysts, 83, 387-528, 531 
Statolith, 388, 390 
Stamen, 123, 124 
Steapsin, 462 
Steganopodes, 650 
Stem, 34, 36 

section of, 10, 12, 13 
"Stemless" plants, 107 
Stems of biennials, 107 

of climbers and trailers, 108 

storage, 112 

structure and function of, 72-78 
Sterculia, 28 

Sterigma a stalk (of a spore). 
Stigma (of plants), 125, 127 
Stigmata (of insects), 478, 574 
Sting, 590 
Stirrup, 403 
Stolon, 109 

Stoma, pi. stomata, 15, 17, 80 
Stomach of crayfish, 458 

of insect, 579 
Stomata of aquatics, 188, 189 

function of, 84 
Stomatopoda, 561 
Storage stems, no 
Stork, 650 
Stormy petrel, 650 
Streptococcus, 231 
Strep toneura, 600, 60 1 
Striate muscle fibres, 414 

Striges, 650 

Strobila a chain of segments or indi- 
viduals formed by transverse 
division of the parent organ- 
ism, 124, 755 

Strobila of microstomum, 124 
Struggle for existence, 816 
Struthiomorphae, 650 
Sturgeons, 633 
Style, 125 

Suberized cell walls, 97 
Subterranean stems, 202 
Suctoria, 516 

Sugar, C 6 H 12 6 , Ci 2 H 22 O n , 90 
Superior (of the ovary) attached 

above the calyx. 
Susceptibility, 774 
Suspensor, 289, 290, 294, 709 
Swallows, 650 
Swamp plants, 192 
Swarm spore a ciliated, motile spore. 

conjugation of, 693 
Sweat glands, 352 
Swine, 666 
Symbionts, 216 
Symbiosis, 745, 746 
Symmetry, 314 

of echinoderm larvae, 320 

of gastropods, 319 

universal, 317 
Synergids, 299 
Syphostoma, 528 
Syrinx, 647 

Tachyglossus. See anteater. 
Tadpole of tunicates, 621 
Taenia a tapeworm. 
Tapeworm, 233, 234 755 
Tapir, 666 
Taste, 366 

buds, 77, 371 
Taxonomy, systematic classification of 


Taxonomic series, 790, 791 
Teeth, 348 

of mammals, 651 

origin of, 721 
Tegmen, 42 
Teleostei, 636 
Teleutospores, 749 
Temperature and vegetation, 196-203 

control, 840, 841 

sense, 364 

of amoeba, 356 
Tendons, 429 

origin of, 721 

Numbers refer to paragraphs; Black Face numbers refer to figures. 



Tendrils, 114 
Tentacles, 528 

Termes a genus of termites. 
Terminal bud, 55 
Termite queen, 260 
Termites, 223, 224, 583 
polymorphism in, 740 
Terrapin, 643 

diamond back, 215 
Testa, 42 

Testes of hydra, 121, 500 
Testudinata, 643 
Tethyodea, 621 
Tetrabranchiata, 616 
Tetrads (in maturation), 695 
Tetraspores four spores produced 

asexually in one mother cell, 


Texas cattle fever, 771 
Thaliaceae, 623 
Thallophytes, 177, 224-263 
Thoracic duct, 472, 473 
Thoracostraca, 561 
Thorns, 113, 114 
Thousand legs, 574, 575 
Thread worms, 540, 756-759 
Thrushes, 650 
Thymus gland, 720 
Thyroid gland, 720 
Ticks, 571 
Tigers, 663 
Tillandsia, 45 
Tinamiformes, 650 
Tissues, origin of, 719 
Titmouse, 650 
Toad fish, 636 
'Toads, 637, 640 
Torpedo, 874 
Tortoise, 643 

^shell, 347, 643 
Toxin, 776 
Tracheae, n, 569, 574, 579 

of insects, 162, 478 
Tracheids, n, 74, 296 
Traguloidea, 666 
Translocation of food substances, 92, 


Transpiration, 86 
Trap-door spider, 154-156 
Tree toads, 640 

Trematoda Trematodes, 536, 753 
Trichinella, 757 
Trichogyne, 248 
Trochophore, 609 

larva of annelids, 501 
Tropaea. See luna moth. 

Trunk fish, 419, 636 
Trypanosome, 239, 766, 767 
Trypsin, 462, 463 
Tryptic having the proteolytic action 

of trypsin. 
Tsetse fever, 766 

fly, 766 
Tube feet, 550 
Tuber, no 
Tuber, 47 
Tuberaceae, 260 
Tubinares, 650 
Tunic, 619 
Tunicata, 619 
Turbellaria, 535 
Turgid, distended, swollen. 
Turkey, 650 
Turtles, 643 

shell of, 420 
Tylopoda, 666 
Tympanum eardrum, 397, 401 

Ulothricales, 241 
Umbel, 119 
Ungulata, 652, 666 
Urea, COH 4 N 2 , 492 
Uredospores, 749 
Ureter, 492 
Urinary bladder, 492 
Ursidae, 663 
Urochorda, 619 
Urodela, 172, 639 
Uropygal gland, 350 
Utriculus, 393, 394 

Vacuole, 672 
Vascular bundle, 74 
Variation, 781 

Variety subdivision of a species, 781 
Vegetation and climate, 223 
Veliger, a mollusc larva of peculiar 
form with a ciliated collar, 

Ventral, 312 

ganglionic chain ventral nerve 

nerve cord, 438, 580 
Vertebral column of vertebrates, 792 
Vertebrata vertebrates, 480, 626 
Vertebrate appendage, 413, 799 

eye, 379 
Vertebrates, circulation in, 472 

development in, 506 

digestive tract of, 460 

exoskeleton of, 419 

kidneys of, 492 

Numbers refer to paragraphs; Black Face numbers refer to figures. 



Vertebrates, locomotion in, 413 

nervous system of, 444 

reproduction in, 506 

segmentation of, 336 

sense of smell of, 370 
of taste of, 370 
organs of, 362 
Vessels (of plants), 74 
Vestigeal organs, 794- 
Vibrio, 227 
Viffi, 472 
Visceral sac, 595 
Vision, 383, 384 
Vitreous humor, 382 
Viverridae, 663 
Voice, 396 
Volvocales, 239 
Vultures, 650 

Walking stick insect, 265 

Walrus, 664 

Warblers, 650 

Warm blooded animals, 840 

Wasps, 590 

care of young, 855 
Water and vegetation, 183-195 

boatmen, 591 

bugs, 591 

striders, 591, 763, 766 

vapor, transpiration of, 84 
Weasel, 663 
Web of spider, 569 
Whales, 652, 665 
Whale bone, 250, 347 

rudimentary limbs of, 795 
Wheat rust, 749 
"White ants" termites, 583 
White blood-corpuscles leucocytes, 

494, 7?6 
Numbers refer to paragraphs; Black 

Whip-poor-will, 650 
Whorl three or more leaves, or other 
parts, set around the same 

"Witches broom," 261 
Wolves, 663 
Wood lice, 561, 563 

pecker, 650 

Worker (ants or termites), 583 
Worms, 349, 542 

connective-tissue of, 416 

digestion in, 455~4S7, 466 

eyes of, 376 

glands of, 349 

integument of, 340 

nephridia of, 490 

sense organs of. See nereis. 

sensitiveness to light, 376 

Xerophytes, 183, 193-195 
Xiphosura, 565 

Xylem, the wood portion of a vascular 

Yeast, 262 

Yolk, effect on cleavage, 711 
Young, care of, 854 
Yucca, 852 

pollination of, 257, 258 

Zooglea, 228 
Zygnemaceae, 236 
Zygomycetes, 250 

Zygospore, a spore formed by the 
fusion of two similar gametes, 


Zygote, a body formed by the fusion of 
two gametes. 

Face numbers refer to figures. 




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General Library 

University of California